Cut a 1/4-inch slice off the stem end of tomatoes. Using a spoon, scoop out the pulp of each tomato, leaving about a 1/4-inch shell. Sprinkle cavities with salt and black pepper. Turn upside down on a paper towel-lined baking sheet to drain; let stand for 30 minutes. Meanwhile, lightly coat twelve 2-1/2-inch muffin cups with nonstick cooking spray; set aside.
Preheat oven to 350 degree F. In a medium bowl use a whisk or fork to combine the sour cream, milk, eggs, and melted butter. Using the whisk or fork, mix in the crackers, flour, sugar, baking powder, and 1/4 teaspoon salt until smooth. Stir in corn and onion. Place tomatoes, cut sides up, in the prepared muffin cups. Fill each with about 3 tablespoons of the corn mixture. Sprinkle each with some cheese.
Bake, uncovered, for 25 to 30 minutes or until filling is puffed and set. Remove from oven; cool in pan on a wire rack for 10 minutes. Remove from cups using a large spoon. Serve warm. If desired, trim with fresh dill sprigs. Makes 12 side-dish servings.