Lamb Burgers with Feta Tzatziki Spread

Name: Lamb Burgers with Feta Tzatziki Spread
Ingredients: Extra-virgin olive oil; 2 red onions, 1 cut into 1/4-inch dice and 1 sliced, for garnish; Kosher salt; Pinch crushed red pepper; 2 cloves garlic, finely smashed; 1 1/2 pounds ground lamb; 2 sprigs fresh oregano, finely chopped; 1/2 bunch fresh dill, finely chopped; 1/2 bunch fresh mint, finely chopped; Zest of 1/2 lemon; 4 whole wheat pitas or 4 seeded hamburger buns; Feta Tzatziki Spread, recipe follows; 1 beefsteak tomato, sliced, for garnish; 2 cups baby spinach, for garnish; 2 cups plain Greek yogurt; 1/2 cup crumbled feta; 1 tablespoon white wine vinegar; 2 cloves garlic, smashed and finely chopped; 2 sprigs fresh mint, leaves cut into chiffonade; 1 small bunch fresh dill, finely chopped; 1/2 English cucumber, coarsely grated