1 ½ lb. ground lamb
2 spring onions or 4 thin scallions, finely chopped
Kosher salt and black pepper
½ cup fresh mint leaves (a fat handful), finely chopped
¼ cup fresh dill sprigs, finely chopped
About 2 tbsp. fresh oregano or marjoram leaves, finely chopped
About 1 tbsp. plus a drizzle of EVOO
About 3/4 cup sun-dried tomatoes (from a bag, not packed in oil)
1 cup walnuts
1 cup crumbled feta (preferably Greek)
1 lemon, juiced (about 1/4 cup)
1 clove garlic, crushed
1 tsp. dried oregano
⅓ to 1/2 cup whole milk
4 brioche rolls, split
Little Gem lettuce, lamb’s lettuce (a.k.a. mâche), or baby spring greens, for topping
In a medium bowl, season the lamb and spring onions with kosher salt and pepper. Mix in the fresh herbs and about 1 tbsp. oil. Form into 4 patties (thinner in the centers for even cooking). Refrigerate for 1 hour
In a small bowl, cover the sun-dried tomatoes with very hot water. Let stand until softened, 10 to 20 minutes. Drain and thinly slice the tomatoes.
In a small skillet, toast the walnuts over medium heat, stirring occasionally, until fragrant and toasted, about 5 minutes. Let stand until cool, about 5 minutes. In a food processor, pulse the nuts just until finely chopped (but not to a paste). Add the feta, half the lemon juice, the garlic, dried oregano, and a drizzle of oil. With the machine running, gradually stream in the milk until a thick sauce forms. Season the feta-walnut sauce.
Heat a cast-iron skillet over medium-high. Add the patties. Cook, turning occasionally, for about 6 minutes for pink centers or about 8 minutes for medium-well. Douse the patties with the remaining lemon juice.
Place the burgers on the roll bottoms. Top with the sun-dried tomatoes and lettuce. Slather a layer of the feta-walnut sauce on the roll tops and set in place.