Preheat your oven to 350°F (175°C) and grease a large rimmed sheet pan with olive oil or non-stick spray.
In a large bowl, whisk together the eggs, salt, pepper, and chopped spinach. Pour the mixture into the prepared sheet pan.
Bake the eggs for 12-15 minutes until just set. Remove from the oven and let them cool completely in the pan.
While the eggs bake, cook the turkey bacon or beef sausage patties in a skillet over medium heat until fully cooked and browned. Transfer to a paper towel-lined plate and let cool completely.
Once cooled, cut the sheet pan eggs into 12 equal portions using a knife or a circular biscuit cutter.
Assemble the sandwiches by placing one egg portion, one slice of cheese, and one protein (turkey bacon or beef sausage) onto each English muffin or bagel.
Wrap each sandwich individually in parchment paper or aluminum foil to prevent freezer burn and condensation.
Place the wrapped sandwiches into a freezer-safe Ziploc bag and store for up to 3 months.
To reheat from frozen: Remove wrapping and microwave for 1-2 minutes, or for a crispier texture, heat in an air fryer at 350°F for 5-8 minutes.