Preheat the oven to 375°F (190°C).
Lightly grease a 9×13-inch sheet pan with olive oil.
In a large skillet over medium heat, cook the breakfast sausage, breaking it up with a spoon, until browned and cooked through, about 6-8 minutes. Remove the sausage from the skillet and set aside.
In the same skillet, add the diced bell pepper and onion. Sauté for 4-5 minutes until softened. Add the chopped spinach and cook for an additional 1-2 minutes until wilted. Remove from heat.
In a large mixing bowl, whisk together the eggs, whole milk, salt, black pepper, garlic powder, and dried thyme until well combined.
Spread the cooked sausage evenly across the bottom of the prepared sheet pan.
Distribute the sautéed vegetables evenly over the sausage layer.
Sprinkle the shredded cheddar cheese evenly over the vegetables.
Pour the egg mixture evenly over all the ingredients in the sheet pan.
Bake in the preheated oven for 30-35 minutes, or until the quiche is set in the center and a toothpick inserted comes out clean.
Remove from the oven and let cool for 5 minutes before slicing into squares.
Serve warm and enjoy!