Preheat the oven to 350ºF and grease a 12×18 inch rimmed sheet pan with non-stick cooking spray.
In a large bowl, whisk together the eggs and add in the milk, pesto, fresh basil, salt, and pepper. Whisk to combine. Carefully pour the egg mixture into the prepared pan and scatter the tomatoes over the top and set aside.
In a small bowl, mix together the ricotta cheese and Parmigiano Reggiano then, using a small spoon, dot it over the top of the frittata.
Carefully place the pan in the preheated oven and bake for 30 to 35 minutes or until the frittata is puffed up, golden brown, and set in the centre.
Allow the frittata to cool for at least 5 minutes before cutting into squares and serving. If you want to get ahead before the big day, this frittata is perfect cold from the fridge!