Averie Cooks

-1 Preheat oven to 400F and to a 9×13-inch casserole dish or baking pan, arrange the potatoes upright in the pan along one of the long edges. Repeat with the remaining potatoes until you have three columns of potato slices; set aside. Tip - If you have a mandoline, use it here!
-1 To a saucepan, add and melt the butter, add the onion and garlic, and cook for 5 minutes; stir frequently.
-1 Add flour, and cook for another minute, whisking nearly constantly, or until the flour is lightly golden browned. This is what’s called a ‘roux’ and is important so that later on the mixture properly thickens.
-1 Add the salt, seasoning blend, pepper, broth, cream, and stir until somewhat thickened, about 3 minutes. 
-1 Turn off the heat and stir in 1 cup each of the Colby Jack and cheddar cheeses and 1/2 cup of the Parmesan; set aside momentarily.
-1 Spread the sour cream over potatoes and pour the cheesy mixture evenly over potatoes.
-1 Evenly sprinkle the remaining one-half cup of each of the three cheeses over potatoes.
-1 Cover potatoes with foil and bake for 1 hour, remove from the oven, and check for doneness. The potatoes should be fork tender and not at all hard. If they're not cooked through yet, recover with the foil and bake for an additional 10 to 15 minutes, or as long as necessary so they cook through.
-1 After the potatoes have cooked through completely, remove and discard the foil, and allow the potatoes to bake uncovered for an additional 30 minutes, or as desired to brown the cheeses. In the final moments of baking the potatoes uncovered, keep a close eye on them so you know exactly how fast the cheese is browning**
-1 Allow potatoes to cool momentarily before serving, about 5 minutes.