Heat oven to 300°F (convection) or 325°F (conventional). Butter a 2-1/2-inch deep half size pan (10"x12") or 2-quart casserole, or twelve 8-ounce ramekins.
Peel potatoes and slice 1/16-inch thick with a mandolin. Place slices in cold water while working. Meanwhile, bring a large pot of water to a boil.
Add sliced potatoes to boiling water. When water returns to a boil, cook 1 minute or just until potatoes are translucent. Drain and rinse with cold water. Spin dry in a salad spinner or pat dry between towels.
Cook sausage over medium-high heat until browned and most of the fat is rendered. Drain on paper towel.
Combine cheeses and salt. Mix well.
Shingle 1/3 of the potatoes evenly in pan, top with 1/3 of the cheese and half the sausage, onion and jalapeño. Repeat layers. End with remaining 1/3 of potatoes and cheese. (Or portion layers in ramekins.) Pour cream evenly over potatoes to cover at least ½-inch depth from bottom of pan.
Cover with foil, tenting if needed to keep from touching the cheese.
Bake 1-1/2 to 2 hours (about 45 minutes for ramekins), until barely bubbling on the edges and potatoes are tender-firm. Test by inserting a pick in center. Remove foil and bake about 20 minutes until golden brown.
Cool 15-20 minutes before serving. Leftovers can be reheated.