Cheesy Scalloped Potatoes and Ham

Preparing the Cheese Sauce:

1. Shred the cheddar and Gruyere cheese and set aside, it should be close to room temperature when mixed into the sauce. **Reserve 1/2 cup of the cheddar to top the casserole with.
2. Melt the butter in a medium saucepan over medium heat. 
3. Add the sliced onions and cook onions for 20 minutes (or more), stirring occasionally. They should begin to brown and caramelize.
4. Add the minced garlic and cook for 1 minute.
5. Gradually add the flour, and mix constantly to form a paste. Heat for 1 minute.
6. Temper the half and half/milk by heating it in the microwave for 30 seconds. Add it to the saucepan and stir to combine.
7. Add the 3/4 tsp of rosemary, then 1/2 tsp each of oregano, thyme, and sage. Simmer for 5 minutes. Remove from heat.
8. Gradually sprinkle in 1 + 1/2 cups of the shredded cheddar and 1/2 cup of the Gruyere until smooth and well combined.
9. Mix in the ham, you may want to reserve some to top the casserole with.

Assemble the Casserole

1. Preheat the oven to 400 degrees.
2. Peel the potatoes and slice them thinly, about 1/8 inch thick. (A mandoline slicer makes this much easier.)
3. Lay 1/3 of the potatoes in a single layer, overlapping each other, on the bottom of a lightly greased 9 x 13 casserole dish. Season lightly with salt and pepper if desired.
4. Spoon 1/3 of the cheese sauce over the potatoes and use a pastry brush to completely cover the potatoes with the sauce, to retain their color.
5. Repeat twice more until all of the potatoes and sauce are layered in the casserole dish. 
6. Top with remaining cheddar cheese and any reserved diced ham.
7. Bake, covered, for 30 minutes.
8. If making ahead: Let it cool completely and freeze or refrigerate until ready to serve. See notes.
9. Remove cover. Add 1/4 cup finely shredded Parmesan.
10. Reduce heat to 375 and bake for an additional 25 minutes, until the cheese is hot, bubbling, and begins to brown.
11. Top with chopped parsley and let it sit for 5 minutes prior to serving.