Peel potatoes and slice crosswise into approximately 3/16" slabs. Keep in cold water until needed to prevent graying.
Peel and dice the onion into very small pieces.
In a medium-sized saucepan, melt 4 tbs. of the butter over medium heat and add the onions. Sauté onions until they are soft. Continue until they become pasty. Add a small amount of water and sauté again until the water is cooked off but be careful not to brown or burn the mix.
Add the cream, 1 cup of the cheese, salt, and white pepper. Crush the bouillon cube into the mix. Stir over medium heat until all ingredients are blended in, add flour (or corn starch) and continue to mix over medium heat until it thickens slightly.
Preheat oven to 375°F, place the remaining 4 tbs. butter into a large, shallow casserole, and melt the butter in the casserole in the oven. Remove the casserole and shake the melted butter around to cover the entire dish.
Place about half of the sliced potatoes evenly around the casserole dish. Add about half of the mix from the saucepan to evenly cover the potatoes, add the remaining potatoes, remaining sauce, and then sprinkle the remaining cheese evenly on the top.
Cover the casserole, place in the oven, and cook covered for about 30 minutes. Check potatoes after this time to see if they have cooked to a slightly soft texture. If not, then cook covered for 10-15 more minutes. After this, uncover and cook until top is golden/dark brown.