Heat oven to 350°F; butter 8-inch-square metal pan. Whisk eggs, milk, spinach, salt, and pepper evenly.
Pour into pan; bake 20 to 25 minutes until center set and 160°F. Rest 5 minutes; cut 6 rectangles.
Refrigerate egg pieces in shallow container within 2 hours; chill fully. Lightly toast bagel halves; cool.
Put one cold egg piece and one provolone slice in each bagel. Wrap each in parchment, then foil.
Bag assembled sandwiches, label, and freeze up to 2 months.
Remove foil from one frozen sandwich; wrap in damp paper towel. Microwave at 50% power 2 minutes; turn and heat on high 1 to 2 minutes until egg center reaches 165°F.
Rest 1 minute. If desired, remove all paper and crisp in 350°F toaster oven 3 to 5 minutes. Serve one per portion.