In a skillet on medium- high heat, melt 2 Tbsp butter, add onion, and cook 3 minutes or until soft and translucent. Add Leeks and stir until golden brown. Remove, set aside, and allow to cool slightly.
In a large bowl, whisk eggs, crème fraiche, garlic, and salt. Mix well.
In a saucepan on medium heat, add milk. Once sides start to bubble, remove from heat. Slowly add milk in egg mixture, whisking at same time.
In an oven dish, add 2 Tbsp butter, layer sweet potato veggie sheets, add salt and pepper.
Add 1/2 Cup egg mixture over sweet potato veggie sheets, then add ½ Cup leek mixture and cheese. Repeat for each layer. (Yields 3 Layers in large casserole Dish and 6 Layers in a small casserole dish).
Cover with foil and bake for 40 minutes. Remove foil and bake for additional 25 minutes, until golden brown, and cheese melted