Open-Faced Flat Iron Steak Sandwich with Caramelized Onions

Illustration generated by AI. The recipe is hand-curated and checked by MealPrep's allergen, diet and nutrition rules.

American · 55 min · high-protein

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Ingredients

• 3/4 lb flat iron steak
• 2 slices sourdough or ciabatta, cut into thick slices
• 1 large yellow onions, thinly sliced
• 2 oz Gruyere or provolone
• 1 1/2 tbsp unsalted butter
• 1/4 cup mayonnaise
• 1/2 tbsp Dijon mustard
• 1 cup arugula
• 1 tsp kosher salt
• 1/2 tsp black pepper
• 1/2 tbsp olive oil

Method (tap to check — screen stays awake)

1. Melt 2 tbsp butter in a wide skillet over medium-low, add the onions and a big pinch of salt, and cook 25 to 30 minutes, stirring every few minutes. Add a splash of water whenever the bottom threatens to catch. They are done when jammy and mahogany, not merely soft — this is not a 10-minute job.
2. Meanwhile salt the steak and leave it at room temperature. Stir the mayonnaise with the Dijon and a grind of pepper.
3. Scrape the onions out. Wipe the pan, raise the heat to high with the olive oil, and sear the flat iron 3 to 4 minutes per side to an internal 128F for medium-rare. Rest 10 minutes on a board.
4. Toast the bread under the broiler with the remaining butter until the edges crisp, about 2 minutes.
5. Slice the steak thinly against the grain, catching any board juices.
6. Spread the mustard mayonnaise on the toast, pile on the onions and steak, drape with cheese, and run the sandwiches back under the broiler for 90 seconds until the cheese slumps. Top with arugula and the board juices.

Nutrition figures are estimates. Verify allergens on every ingredient label before cooking.

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