Over medium-high heat, add the chicken apple sausage and saute until browns all over. Once done, reserve the sausage.
Lower the heat to medium, add a little olive oil to the pan, and saute the garlic and onion until they are translucent and slightly golden in color. Remove the garlic and shallots from the pan and set aside.
Add some more olive oil to the pan, then add the spinach in bunches and saute until it cooks down. Once cooked, set aside.
Preheat the oven to 375 degrees
Whisk the eggs, a pour of whole milk, salt, and freshly cracked black pepper together in a bowl. Set aside.
Grease a sheet pan (12x18 inches) with butter, then spread all of the prepped ingredients evenly throughout the pan.
Pour the eggs over the sheet pan until it is evenly distributed.
Add the grated cheese, making sure to submerge some of the cheese in the egg mixture, so it is evenly distributed.
Put the sheet pan in the oven and bake until it is set ~30-45 minutes.