Name: Ultimate Onion-Mustard Smash Burger
Prep time: 30 min
Total time: 30 min
Yield: 2 servings
Ingredients: 250g (about 1/2 lb) ground beef (80/20 blend); 2 soft burger buns; 1 medium yellow onion, thinly sliced; 2 tsp yellow mustard, plus extra for serving; 2 slices sharp cheddar cheese; 1 tbsp unsalted butter; 1 tbsp neutral oil (like canola or vegetable); Salt and freshly cracked black pepper; 2 leaves crisp lettuce; 2 slices ripe tomato

Ultimate Onion-Mustard Smash Burger

Peter

This smash burger is all about bold, crave-worthy flavor and irresistible texture. The beef patties are pressed thin on a hot skillet, creating a deeply caramelized crust that’s loaded with umami. Sweet, golden onions are cooked right into the beef, while a swipe of tangy mustard sears onto the patty for a signature zing. Melty cheddar, buttery buns, and classic toppings make every bite juicy, crunchy, and perfectly balanced—no ketchup or pickles in sight. It's a burger that celebrates simplicity and technique, delivering a diner-style experience right at home.

Ingredients

Instructions

Divide the ground beef into two equal balls. Do not overwork; keep them loosely packed.
Heat a large cast iron skillet or heavy pan over medium-high heat. Add the oil.
Once hot, add the sliced onions and a pinch of salt. Sauté for 3-4 minutes until golden and softened. Push onions to the side of the pan.
Increase heat to high. Place beef balls onto the skillet. Immediately smash each ball flat with a sturdy spatula or burger press until very thin (about 1/2 inch thick). Season generously with salt and pepper.
Spread 1 tsp mustard on the raw side of each patty.
Top each patty with a portion of the sautéed onions, then flip after 1-2 minutes, once a deep brown crust forms.
Place a slice of cheddar cheese on each patty. Cover the pan briefly to melt the cheese (about 30 seconds).
While cheese melts, butter the buns and toast them cut side down in a separate pan until golden.
Assemble burgers: Bottom bun, lettuce, tomato, cheesy onion-mustard patty, extra mustard if desired, then top bun.
Serve immediately for maximum crispness and juiciness.