Preheat oven to 375 F. Line 9 x 13 inch glass or metal dish with parchment paper, leaving some overhang.

In large bowl, beat eggs. Whisk in spinach and milk. Pour into prepared pan. Bake for 20 to 25 minutes, or until toothpick inserted in center comes out clean. Cool eggs completely. Leave oven on.

Place English muffin halves on large baking sheet, cut-side up. Toast in oven for 5 minutes. Turn oven off.

Remove eggs from pan using parchment overhang. Using cookie cutter or rim of a glass roughly the same size as English muffin, cut 6 egg rounds. Save scraps for extra breakfast sandwiches, salads, stir-fries, or pasta. Top 6 English muffin halves with 1 egg round per half, sprinkle cheese on each, and top with other half of muffin. Tightly wrap each sandwich in parchment paper and place in single layer in reusable container. Freeze overnight, or for up to 1 month.

To reheat, preheat oven to 350 F. Place 1 or more frozen sandwiches on baking sheet. Bake from frozen for 25 to 30 minutes.