LAMB-BACON BURGERS WITH SPICY AIOLI

Steps:

• Aioli:
• Whisk egg yolk and mustard in a medium bowl to combine. Whisking constantly, gradually drizzle in oil, drop by drop at first; keep going until aioli is thickened and smooth. Whisk in chipotle, garlic, lemon zest, and lemon juice; season with salt. Cover and chill.
• Do ahead
• Aioli can be made 1 day ahead. Keep chilled.
• Assembly:
• Preheat oven to 225°. Toss tomatoes, thyme, and 1 tablespoon oil on a baking sheet; season with salt and pepper. Arrange tomatoes cut side down and roast until skin is slightly browned and beginning to separate from flesh, 75-85 minutes. Let cool, then slip off skins.
• While tomatoes are roasting, freeze bacon until almost frozen through, about 40 minutes (this will help it chop cleanly in the food processor). Pulse bacon in a food processor, scraping down sides of bowl as needed, until very finely chopped, about 1 minute.
• Transfer bacon to a medium bowl and gently mix in ground lamb, oregano, sage, and 1 1/2 teaspoons salt just to combine. Divide meat into 4 equal portions and gently shape into 3/4"-thick patties.
• Heat a grill pan or a cast-iron skillet over medium-high. Cook patties until lightly charred and medium-rare, about 2 minutes per side. Generously spread buns with aioli and build burgers with patties, roasted tomatoes, and mozzarella.
• Toss arugula in another medium bowl with vinegar and remaining 2 teaspoons oil; season with salt and pepper.
• Serve burgers with arugula alongside.