Start by slicing each croissant in half horizontally without cutting all the way through. You want a pocket to hold the filling. Place them gently in a baking dish. This keeps them stable while stuffing and baking.
In a bowl, whisk together eggs, milk, salt, and pepper until smooth. Melt butter in a skillet over medium heat and cook the egg mixture slowly. Stir gently until soft and slightly creamy, then mix in cooked sausage or bacon and veggies.
Carefully spoon the egg mixture into each croissant. Make sure to distribute the filling evenly. Sprinkle shredded cheese on top of each stuffed croissant for that gooey, melty finish.
Preheat your oven to 350°F (175°C). Place the baking dish in the oven and bake for about 10–15 minutes. The cheese should melt beautifully, and the croissants will turn slightly crisp on the outside.
Remove from the oven and let them cool slightly. Serve warm for the best taste and texture. The combination of flaky croissants and savory filling is simply irresistible.