For less spicy quesadillas, reduce the amount of jalapeno and cayenne peppers.
To make the salsa, stir together tomatoes, zucchini, yellow pepper, scallions, jalapeno, minced coriander, lime juice and zest, cayenne pepper, and salt in a small mixing bowl.
Heat a medium nonstick skillet over moderate heat and warm a tortilla in it for 1 to 2 minutes. Turn tortilla and spoon half of the salsa over half of it. Top salsa with half the cheese, fold tortilla over filling, and cook until salsa is heated through and cheese has melted, 2 to 3 minutes. Transfer the quesadilla to a plate and keep it warm in an oven at low heat. Repeat with another tortilla. Serve, topped with yogurt and coriander sprigs.