Step 1

Preheat the oven to 375 degrees. Spray a 9-by-13-inch baking pan with nonstick cooking spray or grease with butter.

Step 2

Heat 1 tablespoon of the olive oil over medium-high heat. Add the mushrooms and cook, stirring occasionally, until wilted and beginning to brown; about 5 minutes. Add the spinach, 1 teaspoon salt and a few grinds of pepper. Cook until spinach just begins to wilt, stirring occasionally, about 2 minutes. Remove pan from heat and set aside to cool.

Step 3

Whisk the eggs, milk, 1 teaspoon salt and a few grinds of pepper together in a large bowl. Gently whisk in the cooled mushroom mixture, then pour the egg mixture into the prepared pan. Bake until puffed and set, 15 to 20 minutes. Remove from the oven and let cool.

Step 4

Place the English muffins cut-side up on a baking sheet and brush with some butter if using. Bake until slightly toasty, 3 to 4 minutes.

Step 5

Once the eggs are cool enough to handle, use a large round biscuit cutter (about 3 3/4 inches in diameter) or the rim of a juice glass to cut out 6 rounds of egg. (For ways to use the leftover egg scraps, see Cook's Note.) Using an offset spatula or a butter knife, remove each egg round from the pan and place on bottom half of an English muffin. Top each with a slice of Swiss cheese and the other muffin half. Wrap each sandwich individually with aluminum foil, put in a resealable plastic bag and freeze up to 1 month or refrigerate for up to 1 week.

Step 6

To reheat, unwrap a sandwich and place on a paper towel on a microwave-safe dish. Microwave 1 to 1 1/2 minutes, until the cheese is melted and eggs are warmed through. Alternatively, thaw a foil-wrapped frozen sandwich overnight in the refrigerator, then bake in a 425-degree F oven until warmed through and the cheese is melted, about 10 minutes.