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Grilled Flank Steak with Caramelized Onion Sauce

This Grilled Flank steak is simply seasoned, grilled, then topped with  caramelized onion sauce. 

Ah, grilling season! I love you so. I especially love the chance to grill cuts like flank steak, that only taste good to me when cooked on the grill! This is our first flank steak of the season. Trust me, there are many more to come!

I love flank steak because it has intense flavor and is super affordable. We typically rub it in cumin and chili powder, grill it, and top it with fresh corn and tomato relish. YUM! But this time, I wanted to try something different. I spied this grilled flank steak recipe in my local grocery store's mailer, which typically has some pretty great recipes. They used rib-eye, which we have deemed a "sometimes food" due to the high price of beef right now. We decided to use flank steak instead of rib-eye.

Fair warning, this is not my normal weeknight recipe. The sauce, while very hands off, takes some time to simmer. I'd recommend planning this for a special weekend dinner! 

Flank Steak vs. Skirt Steak

I chose flank steak for this recipe. You'll sometimes see flank steak and skirt steak compared, so it might be tempting to try this recipe with skirt steak!

Skirt steak is a long, fatty, very thin cut of meat that is best when marinated. This is often the meat that's used for fajitas. I often say that skirt steak has almost a "gamey" taste to it. 

Flank steak is thicker and has less fat content. 

If you do substitute skirt steak in this recipe, you'll need to reduce the cooking time per side by at least 2 minutes, depending on how thin the cut is. 

Grilled Flank Steak with Caramalized Onion Sauce 7

Ready for flavorful grilled flank steak topped with a decadent sauce? Let's do it!

First, cut the onion into long slices. Add to a skillet with olive oil and butter, and set to cook and caramelize for about 25 minutes until golden brown.

While the onions are caramelizing, start the remainder of the sauce in a small saucepan. You'll add what feels like every condiment in your fridge (ketchup, soy sauce, lemon juice, etc.), but I promise it goes very quick and is worth it! Let the brown sauce simmer for about 25 minutes until it's well reduced and bubbly, then add to the finished caramelized onions.

While I was in the kitchen working on the sauce, my husband got the grill fired up. He rubbed the flank steak with garlic powder and black pepper, then threw it on a super hot grill for about 5 minutes each side. This yielded a steak that was medium rare through the middle, and medium on the thinner end pieces. 

Since flank steak is lean, you do not want to cook it past medium. If you do, it will be tough. 

How Long to Let Steak Rest

Once the steak is done, pull it off the grill and let it rest for 5-10 minutes before slicing it to let the juices re-distribute and lock into the meat. This is a tip that works for all steaks, regardless of how you cook them! If you slice immediately, the juice will run out of the meat. 

Grilled Flank Steak with Caramalized Onion Sauce 8

How to Cut Flank Steak 

After allowing the steak to rest for 5-10 minutes, cut the steak against the grain using a sharpened chef knife. It's important to cut flank steak into thin slices to keep it from being tough to chew. 

Grilled Flank Steak with Caramelized Onion Sauce

Grilled Flank Steak with Caramelized Onion Sauce

Yield: 5-6
Prep Time: 25 minutes
Cook Time: 10 minutes
Total Time: 35 minutes

Grilled flank steak topped with a savory caramelized onion sauce and bleu cheese.


For the steak

  • 2 lbs flank steak
  • ½ tablespoon black pepper
  • 1 ½ tbs garlic salt

For the sauce

  • 1 medium yellow onion
  • 1 tablespoon butter
  • 1 tablespoon olive oil
  • ½ teaspoon salt
  • ¼ teaspoon black pepper
  • ¾ cup ketchup
  • ¼ cup Worcestershire sauce
  • ¼ cup fresh lemon juice
  • ¼ cup balsamic vinegar
  • 3 tablespoon soy sauce
  • 2 heaping tablespoon brown sugar
  • ½ teaspoon garlic powder
  • crumbled gorgonzola or bleu cheese, to taste


For the caramelized onion sauce

  1. Chop onion into long, thin pieces. Add to large skillet with 1 tablespoon butter and 1 tablespoon olive oil. Cook for 25 minutes, stirring every few minutes, until onions are golden brown and soft.
  2. In a separate, small saucepan, add ketchup, salt and pepper, Worcestershire sauce, lemon juice, balsamic vinegar, soy sauce, brown sugar and garlic powder. Bring to boil then reduce heat to low. Allow to simmer for 20 minutes until liquid is well reduced, then stir in finished caramelized onions. Remove saucepan from heat.

For the steak

  1. Remove flank steak from package. Pat dry with paper towels to remove excess moisture. Add garlic salt and black pepper.
  2. Grill over high heat for about 5 minutes each side.
  3. Top cooked steak with finished caramelized onion sauce. If you desire, sprinkle with gorgonzola or bleu cheese.

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Grilled Flank Steak

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Katerina @

Thursday 18th of June 2015

I am loving that sauce! I'll take the pan with me!

Becky | The Cookie Rookie

Thursday 18th of June 2015

Can I come over for dinner? This looks so fab!

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