Prep - In a large skillet, heat oil over medium-high heat. Add leeks and sauté until softened (about 3 to 5 minutes). Add in spinach and tomatoes; salt and pepper to taste. Sauté until spinach is wilted (about 2 to 3 minutes). Transfer mixture to a greased 9-inch pie plate. Sprinkle with cheese; set aside.
Combine - In a large mixing bowl, whisk eggs and milk until well combined. Pour over cheese.
Create - Bake in a preheated 350° F degree oven until eggs are set and knife inserted in center comes out clean (about 40 to 50 minutes).