Preheat oven to 350°F. Spray 4 ramekins with cooking spray.
Heat olive oil in a skillet over medium heat. Dice onion, bell pepper, and zucchini; add to hot skillet. Sauté for 5 minutes or until softened. Remove from heat.
In a mixing bowl, whisk together eggs, milk, cheese, salt, and pepper. Add veggies and stir to combine; pour mixture into ramekins.
Place ramekins onto baking sheet and then place into oven. Bake for 16-18 minutes until eggs are set.
Toast bread and top with butter if desired. Serve frittatas with toast and bananas on the side.