Preheat oven to 375, butter a 2 qt. casserole dish. Prepare and season mash potatoes as you normally do. Stir egg yolks, cheese, and vegetables into potato mixture. Beat egg whites until stiff. Gently fold into potato mixture. Pour into prepared dish. Bake for 30 minutes, or until knife inserted in middle come out clean.
Note: this recipe begs for creativity! Experiment with different cheeses and veggies. Blue cheese is great with finely chopped green beans and pimento – Swiss is good with crumbled, cooked bacon and chives, and smoked Gouda is great with broccoli. Be creative!