In a skillet, melt the butter over medium heat and scramble or fry the eggs until fully cooked.

In the same skillet, add spinach and sauté until wilted.

Slice each croissant in half horizontally.

Spread mayonnaise and Dijon mustard on the inside of each croissant, if using.

Layer cooked bacon, cheese, scrambled eggs, and sautéed spinach inside each croissant.

Close the croissant and place on a baking sheet.

Preheat the oven to 350°F (175°C) and bake the sandwiches for about 10 minutes, or until the cheese is melted and the croissants are warm.