I made this at work for the first time today. Very simple, very greasy, but delicious if that's your kind of thing.
This recipe yields 2 large pans (probably 1" x 12" x 20" each):
14 lb Yukon gold potatoes
2 L whipping cream
1/2 lb butter
1 lb onions, diced
1/3 cup minced garlic
1 lb spinach, chopped
1 bunch parsley, chopped
Salt and Pepper to taste
Prep everything but the potatoes in a large bowl. Then, using a food processor or mandolin, slice the potatoes thinly, and mix them with everything else thoroughly. Spray the pans with oil, and line them with parchment paper, then fill them with the potato mixture evenly. Cover with aluminum foil and bake at 350F for 90 minutes. Check for tenderness. Once tender, bake uncovered for a further 15 minutes.