Fill large pot with cold water. Add potato slices and 1 tsp salt from measuring spoon. Bring to boil over high heat. Parboil 8-10 minutes till tender but firm. Drain colander, set aside.
Heat skillet over medium. Add bacon pieces, cook 5 minutes till golden crispy using wooden spoon. Scoop bacon slotted spoon to bowl, leave fat in pan.
Add sliced onions and garlic to bacon fat. Cook 8 minutes till soft golden with wooden spoon. Stir in cream, nutmeg, salt, pepper from measuring spoons. Simmer 2 minutes thick.
Preheat oven to 400°F. Layer half potatoes in 9x13 baking dish. Spoon half onion cream over even. Scatter half bacon. Layer half Reblochon slices. Repeat layers, end cheese top.
Cover dish foil. Bake 20 minutes middle rack. Uncover, bake 20-25 minutes more till cheese bubbles brown crusty. Rest 5 minutes for set.