Preheat oven to 375 degrees. Use oil or oil spray to grease a square baking dish (8 or 9-inches).
Whisk eggs in a large bowl. Stir in chopped spinach and salt. Pour the egg mixture into the greased pan. Bake until puffed, lightly browned, and cooked through, 15 to 20 minutes. Set aside to cool.
Place split English muffins cut side up on a sheet pan. Bake for 5 minutes to lightly toast. Remove pan from over then place about 2 tablespoons of shredded cheese on each pieces. Bake until cheese is melted.
Cut baked egg into 6 equal pieces. Place an egg square onto the bottom half of each English muffin. Top with bacon, if using, then add the top of the English muffin to create the sandwich. Allow the sandwiches to cool completely before freezing or storing in the fridge.
To freeze, wrap each sandwich tightly with aluminum foil. Refrigerate for up to a week. Freeze for up to a month. From frozen, unwrap sandwich and place on a plate then reheat in the microwave for 2 to 3 minutes. Allow to cool for 2 minutes before eating.