For the yogurt–avocado sauce: In a small bowl, combine all ingredients, mix until smooth, and refrigerate until serving.

For the burgers: Place pistachios in a food processor and pulse until coarsely chopped. Transfer to a bowl. Add arugula to the processor, pulse briefly, and add to the bowl.

Add onion and garlic, process into a paste, then add to the same bowl.

Add ground lamb, 1 tbsp olive oil, ½ tsp salt, and black pepper. Mix well to combine.

With wet hands, shape about 20 small patties (5 cm wide, 2 cm thick, approx. 40 g each).

Heat 1 tbsp olive oil in a large non-stick pan over medium heat and cook the burgers in batches for about 7 minutes, turning once, until golden brown. Add another tbsp olive oil for the next batch if needed.

Keep warm and serve on a large platter with the yogurt–avocado sauce.