Preheat oven to 425. Grease the wells of a standard-sized 12-cup muffin tin with non-stick spray.
In a large bowl, whisk together the eggs and next 3 ingredients. Set aside.
In a large saute pan, heat the olive oil over medium heat. Add the onion and bell pepper and season with the 1/2 teaspoon salt. Saute until the vegetables are softened and onion is slightly golden, about 7 minutes. Stir in rosemary, and saute for 1 minute, until fragrant.
Divide the onion mixture evenly among the muffin tins and top with an even amount of the cheddar cheese in each tin. Evenly distribute the egg mixture among the cups, using a ladle to pour the mixture directly over the onion mixture and cheese. Place a cherry tomato in the center of each well.
Bake for about 10 minutes, or until the frittatas are just set in the center. (Use a butter knife inserted in the center to test for doneness.) Remove from oven and cool for about 5 minutes before removing from pan. (Running a butter knife around each frittata works well to help loosen them from the pan if they are difficult to remove.)