Preheat the oven to 375°F or 190°C and grease a 9×13-inch baking dish. Pat the thawed potatoes dry to remove excess moisture.
Heat the olive oil in a skillet over medium heat. Cook the sausage and onion for 5 to 6 minutes, until lightly browned.
Whisk the eggs, milk, thyme, garlic powder, salt, and black pepper in a large bowl.
Stir in the sausage mixture, potatoes, diced apple, and cheddar. Pour everything into the prepared baking dish.
Bake for 40 to 45 minutes, until the center is set and the cheese is lightly golden.
Rest for 10 minutes before slicing. Cool the remaining portions completely before wrapping them for freezing.