Grease an 8" x 8" casserole dish with butter or avocado oil. Set aside.In a skillet over medium-high heat, cook the sauce until browned, then remove from the skillet into the casserole dish and return the pan to the stove.
In the same skillet with the sausage pan drippings, add the red bell pepper and kale. Season with salt, pepper, and garlic powder and saute for 4-5 minutes until softened.Remove from the skillet and place on top of the ground sausage in the casserole dish.
Let the veggies and meat cool, then top with feta crumbles.
Make the egg custard
In a large measuring cup or mixing bowl, add the eggs, half and half or whole milk, dry mustard, parmesan cheese, salt, and pepper to taste, and whisk until thoroughly combined.
Pour the egg mixture over the top of the cooled sausage and veggies. Top with the shredded sharp white cheddar.To Freeze: Wrap the assembled casserole in plastic wrap, followed by foil, and place in the freezer. Refrigerate overnight to thaw before baking.
To bake
Preheat oven to 350℉. Bake the casserole for 45-50 minutes or until the middle is set and the top is light golden brown. Cover with foil the last 10-15 minutes of cooking. Let cool for 5 minutes before slicing and serving.