Preheat oven to 400F. Lightly grease a large casserole dish with butter or olive oil.
Sauté the mushrooms and garlic with the olive oil over medium high heat for 4-5 minutes, until tender. Set aside.
Slice each croissant in half. Layer with a generous spread of dijon mustard, a slice of Swiss cheese, and a slice of deli ham. Add the top of the croissant back, as if you were making a sandwich. Slice the croissant sandwich in half. Repeat with remaining croissants.
Add the croissant halves into the prepared casserole dish in a single layer.
In a large bowl, whisk the milk, eggs, and Italian seasoning together. Evenly pour over the croissants.
Distribute the mushrooms over the croissants.
(Optional: Add the shredded cheese evenly over the croissants. Add the prosciutto in small bundles in between the croissants.)
Cover the dish with foil to bake for 20 minutes. Bake uncovered for 8-10 minutes. Enjoy warm with Klarbrunn Lemon Sparkling Water for a refreshing complimentary drink!
