Preheat the oven to 350F. Spray a 9 x 13 x 3-inch Pyrex Deep baking dish with oil.
Heat a large skillet over medium heat. Add the oil and shallots and cook until soft, 6 to 8 minutes.
Add the spinach and cook until wilted. Remove from heat.
In a large bowl whisk the eggs, milk, heavy cream, Feta, scallions, tomatoes, salt and black pepper.
Mix in the spinach and shallots and pour into the prepared dish.
Bake until the center is solid and cooked through, about 1 hour 20 minutes.
