AIOLI
¼

cup rinsed salt-packed capers
1

cup mayonnaise
2

tablespoons olive oil
1

teaspoon finely grated lemon zest
3

tablespoons (or more) fresh lemon juice
1

teaspoon Dijon mustard
2

garlic cloves, finely grated
Kosher salt
ASSEMBLY
1½

pounds ground lamb, preferably grass-fed
Kosher salt
4

tablespoons olive oil, divided
3

celery stalks, thinly sliced on the diagonal
½

fennel bulb, thinly sliced; plus ¼ cup chopped fennel fronds (optional)
¼

large sweet onion, thinly sliced
3

tablespoons fresh lemon juice
2

teaspoons crushed dried mint
Freshly ground black pepper
4

potato or ciabatta rolls, toasted
Boquerones (marinated white anchovies; for serving)