Heat the oven to 350°F and butter an 8-inch-square metal pan. Whisk eggs, milk, spinach, salt and pepper until evenly mixed.
Pour into the pan and bake for 20 to 25 minutes until the center is set and reaches 160°F. Let stand for 5 minutes, then cut into 6 rectangles.
Refrigerate egg pieces in a shallow container within 2 hours and chill fully. Lightly toast bagel halves and cool them.
Place one cold egg piece and one provolone slice inside each bagel. Wrap each sandwich in parchment and then foil.
Freeze after assembly: place wrapped sandwiches in a freezer bag, label and freeze for up to 2 months.
Reheat from frozen: remove foil, wrap one sandwich in a damp paper towel and microwave on 50% power for 2 minutes, turn, then heat on high for 1 to 2 minutes until the egg center reaches 165°F.
Rest for 1 minute, then crisp in a 350°F toaster oven for 3 to 5 minutes if desired, removing all paper first. Serve one sandwich per portion.