Pre-heat the oven to 400 degrees and butter a 9x13 casserole dish.
Warm a large skillet over medium-high heat, melt the butter, and brown the sausage. Use a slotted spoon to scoop the browned sausage out of the skillet, leaving the fat behind to cook the veggies in.
Add the sweet potato to the fat in the skillet over medium-high heat, and cook for a few minutes. Sweet potato takes a bit longer to cook than the other veggies, so don’t add the others just yet.
Once the sweet potato has cooked for a few minutes, add the onion, pepper, apples, sea salt, thyme, and pepper, and cook over medium-high heat, stirring occasionally. Cook the veggies for about 7-10 minutes until soft and sweet.
Add the garlic and spinach to the skillet, stir, and cook for one minute. Turn the skillet off, and stir in cooked sausage and the shredded cheese.
Dump the cooked veggie/sausage mixture from the skillet into the buttered casserole dish, and spread the mixture out evenly. Whisk the eggs and milk in a medium mixing bowl and pour over the veggie/sausage mixture.
Bake the breakfast casserole for 25-30 minutes at 400 degrees until the eggs are set, and the top is a pretty golden brown. Let the casserole rest for 5-10 minutes before cutting.