Preheat oven to 350 degrees and spray 4 4-inch ramekins with non-stick spray (or grease with butter or ghee).

In a medium bowl, whisk together the eggs, milk, salt, and pepper.

Divide the bacon evenly among the 4 ramekins. Add the mushrooms to the ramekins and add 1/4 C of spinach to each ramekin. Evenly pour the egg mixture over the top of the spinach (it's ok if it's not completely submerged, just push the spinach down a bit and the egg will rise while baking). Top each frittata with a little extra mushrooms or bacon if desired.

Place the ramekins on a baking sheet and bake at 350 degrees for 20-25 minutes. The sides will be set and the center will still be slightly soft.

Open the bagels and place each frittata on the bagels and top with cheese (1-2 slices per sandwich). Serve immediately.

If you're not serving the immediately, cool the frittatas to room temperature before building your sandwich. When they are cool, build the sandwiches then wrap them in aluminum foil and freeze until ready to use. I defrost them in the fridge the night before and then heat them in the oven while I get ready at 400 degrees (wrapped in the foil) for about 20 minutes or until they're hot all the way through.