How to Make It
Preheat grill or a grill pan to medium-high (about 450°F). Place mushrooms in a food processor, and process until minced, about 1 minute.
Combine mushrooms, ground sirloin, oil, pepper, and 3/8 teaspoon salt in a medium bowl; gently shape into 4 (4-inch) patties, and place on a baking sheet lined with parchment paper.
Stir together cucumber, yogurt, garlic, lemon juice, parsley, and remaining 3/8 teaspoon salt in a small bowl; set aside.
Place burgers on grill grate, and grill, uncovered, to desired degree of doneness, about 4 minutes per side for medium.
Place 2 lettuce leaves on each plate; top each with a burger patty, tomato slice, red onion slice, and 1 heaping tablespoon yogurt mixture.