Portion lamb into 5 balls. Chill until ready to cook.
Make caramelized onion crema:
Cook onions slowly in butter over medium-low heat. Season with a pinch of salt. Once golden, add minced garlic, sugar and sautee for about a minute. Let cool. Blend with sour cream, mayo, salt, and pepper until smooth.
Sauté mushrooms:
In a hot pan, melt butter and a little oil. Add mushrooms. Let them sear for 30 seconds, then stir. Add rosemary, thyme and sage. Cook until golden. Season with salt and pepper.
Cook smash burgers:
Heat a pan over medium. Add oil. Smash each lamb ball flat using parchment paper and a spatula. Season. Cook 30 seconds, flip, season again. Top each patty with brie.
Stack burgers:
Stack two patties together with brie in between. Keep warm in a 350°F oven to melt the cheese fully.
Toast buns:
Butter buns and toast face-down in a pan until golden.
Assemble:
Spread caramelized onion crema on the bottom bun. Add lettuce, the double-patty stack, sautéed mushrooms, more crema on the top bun, and close it up. Enjoy!