Heat the oven to 350°F. Lightly grease an 8-inch square or 9-inch round baking pan, or line the pan with parchment.

Heat the olive oil in a skillet over medium heat. Cook the onion and bell pepper for 4–5 minutes, until softened, then add the spinach and cook for 1–2 minutes, until wilted.

Whisk the eggs, milk, kosher salt, and black pepper in a bowl. Stir in the cooked vegetables after they have cooled briefly.

Pour the egg mixture into the prepared pan and bake for about 15–20 minutes, until the center is set and no longer jiggles. Remove it as soon as it is set to prevent dry eggs.

Cool the egg bake for several minutes, then cut it into 8 portions.

Split and lightly toast the English muffins if desired. Place one egg portion and one cheese slice on each muffin, adding an optional turkey-sausage patty or slice of Canadian bacon if using.

Cool the assembled sandwiches completely before wrapping each one in parchment or foil. Place the wrapped sandwiches in a freezer bag or airtight container and label the bag with the date.