Quick tip:The key to this sandwich is to lightly toast the croissant so that it does not get soggy when adding the sauce, but to not overcook which will dry out the croissant. If ever in doubt, less time is better!
Dress it Down (Classic Style):
1. Stick with the Croque-Monsieur (without egg) and lose the avocado garnishment.
2. Use normal sandwich bread instead of a croissant. Depending on the soggy factor of the bread you use, you may need to toast for a different amount of time before assembling the sandwich.
Change it Up (Substitutions): For the Gruyere, you may substitute normal Swiss cheese. The flavor will be about the same, but the Gruyere does produce a smoother texture. I've done it with Swiss and it still tastes great!