Preheat oven to 350F and butter a 9x13 in casserole dish with 1 1/2 Tbsp butter.
Layer the bottom of the casserole dish with 1/3 of the potato slices. Sprinkle with salt and pepper. Layer on 1/2 of the sliced onions and 1/2 cup of the Swiss cheese. Layer on 1/2 of the bacon, 1/2 of the parsley.Sprinkle with a little of the Parmesan.
Repeat by layering on 1/3 of the potato slices, sprinkle again with salt and pepper. Layer on the remaining sliced onions, 1/2 cup of the Swiss cheese, the remaining bacon, parsley and sprinkle with Parmesean.
Top with the remaining potato slices. Add the half and half. Dot the potatoes with the remaining 1 1/2 Tbsp of butter.
Cover with aluminum foil and bake for one hour. Remove the foil and sprinkle on the remaining Swiss and Parmesean cheese. Return to the oven for an additional 40 minutes.
When done, the potatoes should be tender and liquid mostly absorbed.