Combine the first three ingredients in a small bowl to make the Seasoning Mix.
Bring a large pot of water to a boil over high heat. Add the sliced
potatoes, return to a boil, and cook until just firm-tender, about 2 to 3
minutes. Drain and rinse under cold running water.
Melt 4 tablespoons of butter in a 12-inch skillet over high heat. When it
begins to sizzle, add the onions and cook, stirring occasionally, until the
onions are golden, about 4 to 5 minutes. Stir in 1 tablespoon of the
seasoning mix, the grated potatoes, and the remaining 4 tablespoons butter.
Cook, scraping often with a spatula to keep a crust from forming on the
bottom of the skillet, about 5 minutes. Add 1 tablespoon of the seasoning
mix and 1 cup of the stock and cook, scraping the bottom of the pan
occasionally, until the mixture is thick, light brown in color, and sticking
hard to the bottom of the skillet, about 5 to 6 minutes. Add the bell
peppers, parsley, remaining 1 cup stock, and the remaining seasoning mix,
and cook, stirring occasionally, for 3 minutes. Whisk in the milk, breaking
up the grated potatoes (they will act as a thickening starch). Bring to a
rolling boil and cook, whisking occasionally, until the sauce is light
brown, about 4 minutes. Add the cream and return to a boil, whisking
occasionally. Reduce the heat to low and simmer, whisking occasionally,
until very thick, about 4 to 5 minutes. Add 1 cup of the Cheddar and the
Monterey Jack cheese and cook, whisking, until the sauce returns to a boil.
Remove from the heat.
Preheat the oven to 375o.
Pour a thin layer of the sauce over the bottom of a large, deep casserole
dish, about 10 by 10 by 4 inches. Add the sliced potatoes and pour the
remaining sauce over them. Sprinkle the top with the remaining 2 cups of
grated Cheddar cheese. Bake uncovered until brown and bubbly on top, about