Preheat your oven to 350°F. Slice the croissants in half horizontally and place them on a baking sheet.

In a large bowl, whisk together the eggs, milk, salt, and black pepper until well combined.

Heat a non-stick skillet over medium heat and add a small amount of butter to grease the pan. Pour in the egg mixture and cook, stirring gently, until the eggs are just set but still soft, about 3-4 minutes.

Remove the skillet from heat and stir in the shredded cheddar cheese until melted and well mixed.

Assemble the sandwiches by placing a layer of cooked bacon or sausage on the bottom half of each croissant. Top with a generous scoop of the cheesy scrambled eggs, followed by slices of avocado and fresh spinach or arugula if desired. Place the top half of the croissant on each sandwich.

Bake the assembled sandwiches in the preheated oven for 5-7 minutes, or until the croissants are warm and slightly crispy.

Serve immediately and enjoy your delicious breakfast sandwiches!