Pistachio, Lamb, and Beef Burgers

INGREDIENTS

• 1/2 cup salted roasted pistachios
• 3/4 cup coarsely chopped onion, plus 4 onion slices (1/4 in. thick)
• 1 large egg white, whisked to blend
• 1/2 pound ground lamb
• 1/2 pound ground beef
• 1 tablespoon baharat* (Arabic spice blend)
• 2 teaspoons black pepper
• 4 to 5 tsp. Aleppo pepper*, divided
• 1 1/4 teaspoons kosher salt
• 3 cups cherry or pear tomatoes (any color)
• 1/4 cup mayonnaise
• 1 1/2 teaspoons lemon juice
• 2 teaspoons olive oil
• 4 hamburger buns, split
• Butter lettuce leaves