Pre-heat oven to 350 degrees. Pour 2 tbsp into casserole dish (apx 8x10 in)--warm in over before adding frittata mixture.
Saute onion in remaining olive oil until caramelized. Add onion to all other veggies and parsley. Toss veggies with your hands in large mixing bowl.
Mix eggs and milk. Pour egg mixture over veggies, salt, and pepper. Stir until all veggies are coated with egg mixture. Pour into hot casserole dish and cook for 45 minutes, until egg is set, and the top of veggies are slightly browned.
Mix yogurt and pesto. Garnish frittata with tomatoes, pesto yogurt and chives.