Seed tomatoes and cut into 1/4-inch dice. Quarter avocado, removing pit, and peel. Cut avocado into 1/4-inch dice. In a small bowl stir together tomatoes, avocado, onion, lemon juice, and Tabasco and season with salt and pepper. In another small bowl stir together sour cream, coriander, and salt and pepper to taste. Preheat broiler. Put tortillas on a large baking sheet and brush tops with oil. Broil tortillas on a rack set 2 to 4 inches from heat until pale golden. Turn tortillas and broil until other sides are pale golden. Sprinkle tortillas evenly with Monterey Jack and broil until cheese is melted and bubbling. Spread avocado mixture evenly over 2 tortillas and top each with 1 of remaining tortillas, cheese side down, to make 2 quesadillas. Transfer quesadillas to a cutting board and cut each into 4 wedges. Top each wedge with a heaping teaspoon of sour cream mixture and garnish with coriander sprigs.