Butter the cups of a muffin tin (just the number of stuffed croissants you’re making).
Slice croissants in half crosswise and then cut the small tip off of each end. This will allow it to sit flat in the muffin cups.
Press a hole in the middle of each half of the croissants using a very small cup or small round syrup bottle.
With a small spoon, spread mustard on the bottom and around the sides of each half of the croissants.
Next, tuck in a slice of ham in each croissant.
Add some shredded gruyere cheese and use the small cup or bottle to gently pack in the cheese on top of the ham.
Crack an egg into a cup or small bowl and pour the egg into the crevice you created. Add a pinch of salt.
Place the muffin pan in the oven and bake for approximately 25 to 30 minutes depending on the size of the eggs you use. We used large eggs and baked them for 27 minutes.
Remove the croissants from the oven, carefully take them out of the muffin pan, place them on small plates, and serve immediately. They can also be served at room temperature too!