Heat the oven & prep the dish. Set the oven to 350°F (175°C). Grease a 9×13-inch baking pan so nothing sticks, and get your veggies chopped while the oven warms.

Sauté the veg. Warm the avocado oil in a large Dutch oven or heavy skillet over medium-high heat. Add the onion, bell pepper, and sweet potato cubes. Cook, stirring occasionally, until the pieces start to soften and pick up color (about 8–9 minutes). Stir in the garlic during the last minute of cooking.

Layer the vegetables. Transfer the softened vegetables to the prepared casserole dish and spread them into an even layer.

Brown the sausage. Return the pan to the stove, add the turkey sausage, and cook until it’s no longer pink. Break the sausage into small pieces as it browns. Drain any excess liquid if needed, then scatter the cooked sausage over the vegetables in the casserole pan and toss together.

Whisk the eggs. In a bowl, beat the eggs with the milk, shredded mozzarella, salt, thyme, and pepper until smooth and well combined.

Assemble & bake. Pour the egg mixture over the sausage-and-vegetable layer. Give the dish a gentle stir so the custard seeps into the potatoes and sausage. Place the pan in the oven and bake 40–45 minutes, or until the center feels set and a knife inserted near the middle comes out mostly clean. If the top browns too quickly, loosely tent with foil for the last 15 minutes.

Rest, slice, serve. Remove from the oven and let the casserole rest about 15 minutes; this helps it firm up and makes slicing neater. Serve warm.