In a medium bowl, whisk together the eggs, egg whites, milk and Dijon mustard; set aside.
In a 10 to 12-inch ovenproof nonstick skillet, spray with cooking spray and heat to medium-high.
Saute onion until softened- about 2 minutes.
Add the mushroom, broccoli, cauliflower and zucchini to the skillet. Saute until slightly softened- another 3 to 4 minutes.
Whisk the egg mixture again, then pour over the vegetables.
Sprinkle tomatoes and feta cheese on top.
Place a lid on the skillet, reduce heat to medium and cook until the bottom and sides of the frittata are firm- 8 to 10 minutes.
Preheat the oven broiler.
Place the skillet under the broiler and broil until the frittata is cooked through (no longer jiggly) and slightly browned on top- about 5 minutes (watch closely).
Cut into 4 wedges and serve immediately- serve over a handful of arugula, if desired.