Heat the olive oil in a large skillet set over medium-high heat. Add the sausages and sauté, breaking them down with a wooden spoon, until cooked. Add the bell pepper, green onions, and garlic, and sauté for 3 more minutes. Transfer to a plate and let cool completely to room temperature.
Preheat the oven to 400 °F. Lightly grease a 23-cm square baking dish, then line the bottom and two sides with parchment paper.
In a large mixing bowl, whisk the eggs, milk, parsley, salt, and red pepper flakes or black pepper together. Add the sausage mixture and mix to combine. Pour the egg mixture into the prepared baking dish. Bake for 25 minutes, or until cooked through. Set over a wire rack and let cool completely.
Unmould the eggs, then peel off the parchment paper. Cut into 9 square patties.
Slice open the English muffins, then toast and let cool completely. Garnish each English muffin with a slice of Cheddar cheese. Add a sausage and egg patty, then top with a second slice of Cheddar cheese. Close the sandwiches and wrap individually in parchment paper. Store the sandwiches in a large airtight container or zip-top freezer bag. Freeze the sandwiches.
To reheat, microwave the parchment-paper-wrapped sandwiches one at a time for 1 minute each, then let sit for 5 minutes before eating. For a crisper alternative, warm in a 350 °F oven for 8 to 10 minutes. Let cool for 5 minutes and enjoy.