Preheat a non-stick skillet over medium heat and lightly grease it with butter.
In a mixing bowl, whisk together the eggs, milk, salt, and black pepper until well combined.
Pour the egg mixture into the heated skillet and cook, stirring gently, until the eggs are scrambled and just set, about 3-4 minutes. Remove from heat.
Slice the croissants in half horizontally and place them on a baking sheet.
Layer the bottom half of each croissant with scrambled eggs, followed by two slices of bacon or a sausage patty, and a generous sprinkle of cheddar cheese.
Add avocado slices and fresh spinach or arugula if using.
Place the top half of the croissant on each sandwich and optionally place them back in the skillet to warm through, about 1-2 minutes per side, or until the cheese melts.
Serve immediately and enjoy your delicious breakfast sandwich!