Preheat oven to 350 degrees F.
In a large skillet, heat olive oil and butter over medium heat until melted.
Add red pepper, onion, one teaspoon salt and 1/4 teaspoon pepper and cook until softened, about 5 minutes, stirring often.
Add spinach and garlic cloves and cook until wilted.
Add tomatoes and Kalamata olives and continue to cook for 2 minutes. Remove from the heat.
Place the mixture evenly in the bottom of a 10-inch round baking dish (or similar size).
In a large bowl, whisk together eggs, milk, heavy cream, Dijon mustard, one teaspoon salt, half a teaspoon pepper and oregano leaves until well combined.
Pour the egg mixture over the vegetable mixture.
Bake for 45 minutes, until lightly browned and a knife inserted in center comes out clean. Let cool for 5 minutes. Serve.