Steaming them first seems like unnecessary fuss for both mashed and scalloped. For roasted there’s at least a rationale for crisper outsides.
I par-cook the slices in milk, which will thicken from the potato starch during the process. Transfer to the baking dish, dot with butter, and finish in the oven. (If you’re not fussed about serving, you can use a pan that goes directly into the oven and skip the transfer.)
You can use cream if you prefer – I use whole milk and it’s rich enough for me.
