In a 4.5 Qt saucepan bring 3 quarts of water to a boil. Blanch vegetables by dropping broccoli, cauliflower and summer squash into boiling water and cooking for 2 to 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 frittata pan over medium heat, melt 1 tablespoon butter. Pour in egg mixture. Add blanched vegetables, onions and mushrooms to mixture. Reduce heat and cook until frittata is set, about 10 – 15 minutes. The top of the frittata will still be a little runny. Place shallow pan upside down on top of deep pan, flip egg mixture into shallow 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 SKU / UPC. A Williams-Sonoma exclusive.