Heat olive oil in a skillet over medium heat. Add onion and saute 3 minutes. Add pepper slices and continue to saute 3-5 minutes more until softened. Add spinach and saute until wilted. Mix in mustard and set aside.
In a medium bowl, whisk together eggs, egg whites, milk, thyme, and salt and pepper to taste.
Spray an 8-9" round baking dish with cooking spray. Spread 1/3 of the cheese on the bottom of the baking dish. Top with the sauteed vegetables. Pour egg mixture on top and sprinkle with 1/3 of the cheese. Bake for 20 minutes.
Remove frittata and sprinkle with remaining 1/3 of the cheese. Bake another 10 minutes until frittata is fully cooked and cheese is beginning to bubble and brown.
Remove from oven; let cool five minutes before cutting.