Preheat oven to 350 degrees
In large bowl, break and lightly whip eggs with a fork. Add milk, salt, pepper and mix with fork.
Pour egg mixture into the prepared pan, top with chives, tomatoes and cheese. Bake for about 20-25 minutes or until eggs are set. If you will be freezing to use at a later date then lean on the shorter side so that when you reheat the eggs they won’t be overcooked.
Let the frittata cool for at least 5 minutes, remove to a cutting board and cut into squares with a table knife or use a circle cookie cutter for a pretty presentation and one that will fit perfectly onto an English muffin for a sandwich.
-Freezing - remove from baking pan and place on a baking sheet on parchment paper. Freeze for at least an hour, then place in labeled resealable plastic bags and freeze for up to a month. For reheating, use microwave or oven but err on too little time than too much time, nothings worse than "rubbery" eggs. For a full frittata heat in the oven at 350 degrees for about 15 minutes. For smaller pieces of the frittata that have been cut, reheat in oven for 10 minutes or in the microwave for about 1 minute.