Preheat the oven to 350 degrees. Coat a mini baking dishes with cooking spray.
Heat the olive oil in a sauté pan over medium heat then add the diced onion and sauté, stirring occasionally, for a few minutes. Add the spinach and tomatoes then cook, stirring occasionally, for 2 minutes, or until softened. Add the minced garlic to the pan then season with a pinch of crushed red pepper flakes, sea salt, and freshly cracked pepper, to taste. Cook and stir constantly for 30 seconds. Remove from the heat.
Whisk together the eggs and milk. Season with sea salt and freshly cracked pepper, to taste.
Sprinkle the bottom of the mini baking dish with half of the spinach & tomato mixture then top with extra sharp cheddar.
Add the egg mixture then sprinkle the top with the remaining spinach & tomato mixture and extra sharp cheddar.
Place into the oven and bake for 25-30 minutes, until cooked through. Remove from the oven and serve immediately. Enjoy.