Preheat the oven to 325°F. Line two rimmed baking sheets with parchment paper and set aside.
In a large frying pan, heat the oil over medium-high heat. Add the turkey sausage and cook, without stirring, until the first side is golden brown, 3 to 4 minutes. Flip and cook the other side until golden brown, another 2 to 3 minutes. Add the spinach and cook, stirring and breaking up the meat, until the spinach is just wilted (the sausage doesn’t have to be fully cooked through, as it will finish cooking in the oven). Let cool.
In a large mixing bowl, whisk together the eggs, milk, and salt. Stir in the turkey mixture until well combined. Pour onto one of the prepared baking sheets, spreading evenly.
Bake for 15 to 20 minutes or until eggs are set. Let cool for 10 minutes then cut into 12 squares.
Meanwhile, cut the English muffins in half. Place them cut side up on the other baking sheet. Toast in the oven for the last 10 to 12 minutes of the egg baking time.
Spread butter and/or mayo over both halves of the English muffins. Lay a slice of cheese on one half of each English muffin, followed by a square of egg. If desired, top with avocado and tomatoes. Cover with the other half of the English muffin and enjoy, or see storage instructions in the notes below.
