Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch sheet pan with cooking spray or butter.
In a large skillet over medium heat, cook the ground breakfast sausage, breaking it up with a spatula, until browned and fully cooked, about 6-8 minutes.
Add the diced yellow onion and red bell pepper to the skillet with the sausage. Cook until the vegetables are softened, about 4-5 minutes. Remove from heat and stir in the chopped spinach until wilted. Set aside to cool slightly.
In a large mixing bowl, whisk together the eggs, whole milk, salt, black pepper, garlic powder, and dried thyme until fully combined.
Spread the cooked sausage and vegetable mixture evenly across the bottom of the prepared sheet pan.
Sprinkle the shredded sharp cheddar cheese evenly over the sausage and vegetable layer.
Pour the egg mixture evenly over the sausage, vegetables, and cheese in the sheet pan.
Bake in the preheated oven for 35-40 minutes, or until the quiche is set in the center and lightly golden on top. A knife inserted in the center should come out clean.
Remove from the oven and let the quiche cool for 5-10 minutes before slicing into 8 squares and serving.