In a 4.5-qt saucepan, bring 3 qts. water to a boil.
Blanch vegetables by dropping broccoli, summer squash into boiling water and cooking for 2 - 3 minutes.
Remove and drain vegetables. Immediately cool under cold water.
Whisk eggs together; beat in Parmesan cheese and season with salt and pepper.
In deep half of a nonstick frittata pan over medium heat, melt butter. Pour in egg mixture. Add blanched vegetables, peppers, onions and mushrooms to mixture.
Attach shallow pan upside down on top of deep pan to cover eggs, reduce heat and cook until frittata is set, about 10 - 15 minutes. (The top of the frittata will still be a little runny.)
Using an oven mitt, carefully hold the two handles of the frittata pans together and flip the pan and cook the uncooked side for about 10 minutes.
Slide frittata onto plate. Serve warm or chilled.
Recipe included with Calphalon One Nonstick Frittata Pan, A Williams-Sonoma exclusive.