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French Onion Smash Burger Tacos

French onion smash burger tacos with thin, crispy smash patties, deeply caramelized sweet onions, and melted Swiss or Gruyère in soft flour tortillas. The onions simmer down into a glossy, slightly tangy topping with balsamic and honey for big flavor in every bite.
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Prep Time 20 minutes
Cook Time 35 minutes
Total Time 55 minutes
Servings: 4 servings
Course: Main
Cuisine: French-Mexican Fusion
Calories: 720
Ingredients Equipment Method Notes

Ingredients
  

French onion filling
  • 3 yellow onions Slice thin for faster caramelization.
  • 3 tbsp butter
  • 2 tbsp balsamic vinegar
  • 1 tbsp honey
  • 1 tsp thyme
  • 1 cup beef broth
  • salt and pepper Use to taste; keep seasoning consistent across the tacos.
Smash burger patties and assembly
  • 1.5 lb ground beef Form into 8 small patties (about 2 per person).
  • salt and pepper Generously season the beef patties before smashing.
  • 8 small flour tortillas Warm until pliable in a dry skillet.
  • 8 Swiss or Gruyère cheese Use 1 slice per patty.
  • fresh parsley Chop and garnish for freshness.
  • horseradish mayo Serve on the side or drizzle over the top.

Equipment

  • 1 cast iron skillet
  • 1 sheet pan

Method
 

Caramelize the French onions
  1. Melt butter in a large skillet over medium heat, then add sliced onions and cook, stirring occasionally, for 20-25 minutes until deeply caramelized and golden brown; keep the onions glossy, not scorched, as they darken.
  2. Stir in balsamic vinegar, honey, thyme, and beef broth, then simmer for 5-10 minutes until slightly reduced; the mixture should look thicker and coats the back of a spoon.
  3. Season with salt and pepper, then remove from heat and set aside while you cook the burgers.
Smash burgers, melt cheese, and build tacos
  1. Form 8 small patties from ground beef, then season generously with salt and pepper so each side is evenly speckled.
  2. Heat a cast iron skillet or griddle over high heat until very hot, then smash each patty flat against the hot surface using a spatula and cook for 2-3 minutes per side until crispy edges form; press only once for a thin crust.
  3. Top each smash burger with a slice of Swiss or Gruyère cheese and cover to melt, watching for the cheese to fully soften and flow over the crispy surface.
  4. Warm tortillas in a dry skillet until flexible, using short flips so they don’t blister too long.
  5. Assemble: place one cheesed smash burger patty in each tortilla and top with caramelized onions, then garnish with fresh parsley and serve with horseradish mayo.

Notes

Pro tip: caramelize the onions in stages—if the pan looks dry, lower the heat slightly and keep stirring to prevent bitter browning. Refrigerate leftover caramelized onions and assembled components separately for up to 3 days; rewarm onions in a skillet with a splash of beef broth. Freeze the uncooked smashed patties for up to 2 months, and cook from frozen or thaw overnight in the fridge. For a lighter option, use 90% lean ground beef and swap to a reduced-fat Swiss/Gruyère slice.