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French Onion Smash Burger Tacos

French onion smash burger tacos with thin, crispy smashed patties topped with deeply caramelized sweet onions and melted Swiss or Gruyère cheese in soft flour tortillas. The onions are simmered with balsamic, honey, thyme, and beef broth for a glossy, sweet-savory finish.
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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: 820
Ingredients Equipment Method Notes

Ingredients
  

ground beef
  • 1.5 lb ground beef
salt and pepper
  • 0.25 salt to taste
  • 0.25 black pepper to taste
yellow onions
  • 3 yellow onions large, sliced thin
butter
  • 3 tbsp butter
balsamic vinegar
  • 2 tbsp balsamic vinegar
honey
  • 1 tbsp honey
thyme
  • 1 tsp thyme
beef broth
  • 1 cup beef broth
flour tortillas
  • 8 flour tortillas small
Swiss or Gruyère cheese
  • 8 Swiss or Gruyère cheese slices 8 slices
fresh parsley
  • 1 fresh parsley for serving, chopped
horseradish mayo
  • 0.25 cup horseradish mayo for serving

Equipment

  • 1 cast iron skillet
  • 1 sheet pan

Method
 

Caramelize the French onion topping
  1. Melt the butter in a large skillet over medium heat. Add the sliced onions and cook, stirring occasionally, for 20-25 minutes until deeply caramelized and golden brown, with onions turning dark amber and glossy.
  2. Stir in the balsamic vinegar, honey, thyme, and beef broth. Simmer for 5-10 minutes until slightly reduced, so the liquid coats the onions, then season with salt and pepper to taste.
Smash and melt the burger patties
  1. Form 8 small patties from the ground beef and season generously with salt and pepper. Keep patties uniform so they cook evenly and crisp at the edges.
  2. Heat a cast iron skillet or griddle over high heat. Smash each patty flat against the hot surface using a spatula, cooking for 2-3 minutes per side until crispy edges form and the patties darken in spots.
  3. Top each patty with a slice of cheese and cover to melt. Cook just until the cheese fully melts and turns stretchy over the crisp patty.
Warm tortillas and assemble
  1. Warm the flour tortillas in a dry skillet. Heat for about 20-30 seconds per side until pliable with light browning spots.
  2. Place one smash burger patty in each tortilla and top with caramelized onions. Garnish with fresh parsley and serve with horseradish mayo.

Notes

Pro tip: If the onions start browning too fast, lower the heat to medium-low and keep stirring so they caramelize evenly without burning. Store caramelized onions covered in the fridge up to 4 days; rewarm gently in a skillet. The smashed patties are best fresh, but you can refrigerate cooked patties up to 3 days and reheat in a hot skillet to re-crisp. For a lighter option, use 90% lean ground beef and keep the cheese slices, or swap to reduced-fat Swiss/Gruyère for fewer calories.