Toast the English muffins. Butter the English muffins and spread the smashed avocado evenly over half of the English muffins, set aside.
Cook bacon in a large, 12-inch, nonstick skillet over medium heat until nice and crispy, about 7 to 9 minutes. Transfer to paper towel–lined plate. Break each slice in half when they're cool enough to handle.
Discard all but 1 tablespoon of bacon fat from skillet, and heat over medium-high until shimmering. Add the eggs, and cover and cook for 1 minute.
Top each egg with 3 (halved) pieces of bacon, and 2 Tablespoons of shredded cheddar cheese. Cover the pan, remove it from the heat, and let stand until the cheddar is melted and the egg whites are cooked through about 2-3 minutes.
Place 1 bacon and-cheese-topped egg on each of the English muffins with smashed avocado. Add 2 Tablespoons of spinach to each, then top with tomato slices and muffin tops. Serve.
Cool the assembled sandwiches to room temperature and then wrap them in foil.
Place the foil-wrapped sandwiches inside a freezer-safe Ziploc bag and freeze for up to 4 months.
Preheat oven or toaster oven to 350°F.
Reheat in the oven for 15-20 minutes, or until heated through.