At Rioja, in Denver, half of the bacon is replaced with ground fatback, a step that makes this excellent burger even better.
- 2 teaspoons Dijon mustard
- 1 canned chipotle chile in adobo, finely chopped
- 1 garlic clove, finely grated
- 2 teaspoons finely grated lemon zest
- 1 tablespoon fresh lemon juice
- 4 plum tomatoes, cored, halved lengthwise
- 1 tablespoon thyme leaves
- 1 tablespoon plus 2 teaspoons olive oil
- 1½ teaspoons kosher salt, plus more
- Freshly ground black pepper
- ½ pound thick-cut bacon, cut into pieces
- 1 pound ground lamb, preferably from the shoulder
- 3 tablespoons finely chopped fresh oregano
- 3 tablespoons finely chopped fresh sage
- ⅓ pound fresh mozzarella, thinly sliced
- 1 teaspoon Sherry vinegar or red wine vinegar
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.
Preheat oven to 225°. Toss tomatoes, thyme, and 1 Tbsp. 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 ½ tsp. salt just to combine. Divide meat into 4 equal portions and gently shape into ¾"-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 tsp. oil; season with salt and pepper.
Serve burgers with arugula alongside.
*Raw egg is not recommended for the elderly, pregnant women, children under 4, and people with weakened immune systems.