Preheat the oven to 325°F. Grease a 9 x 13-inch baking dish with butter.
In a medium bowl, whisk together the eggs, milk, and a pinch each of salt and black pepper. Stir in the spinach. Pour the eggs into the prepared baking dish. Bake for 15 to 18 minutes, until set. Remove from the oven, let cool. Use a 3 ½-inch cookie cutter or the rim of a drinking glass to cut the eggs into six rounds.
In a medium bowl, combine the chicken, peppers, paprika, chili powder, onion powder, garlic powder, salt, and black pepper. Use your hands to mix, then divide into 6 balls and flatten into patties.
Add a few patties to the pan. Cook undisturbed until cooked through, 3 to 5 minutes per side. Transfer to a plate and repeat with the remaining patties.
To assemble the individual sandwiches, spread the insides of the English muffins with butter. Layer the bottom of each with an egg round, followed by a chicken patty. Add a slice of cheese and finish with the muffin top.
Wrap the sandwiches in aluminum foil. Store refrigerated for up to 4 days or in the freezer in an airtight container for up to 3 months. To heat from the fridge, remove the foil and microwave for about 2 minutes or bake in the oven at 350°F for about 10 minutes. To heat from frozen, if possible, thaw overnight in the fridge, bake in the foil at 350°F for about 30 minutes.
