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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Coat a 9-by-9-inch metal baking pan with butter (if youre not planning to reheat the casserole on a grill, a ceramic or glass baking dish is OK to use); set aside.
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3
Place the bacon in an even layer on a rimmed baking sheet and bake until crisp and browned, about 15 to 20 minutes.
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4
Remove to a large paper-towel-lined plate and set aside to cool.
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5
Meanwhile, melt the measured butter in a large, wide pot or Dutch oven over medium heat.
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6
Add the bell pepper and onion, season with salt and pepper, and cook, stirring occasionally, until softened, about 5 minutes.
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7
Add the flour and cook, stirring occasionally, until the flour is no longer raw-tasting, about 1 minute.
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8
While whisking constantly, slowly pour in the milk and whisk until smooth.
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9
Let the mixture come to a boil and remove from the heat.
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10
Whisk in the measured salt, sour cream, and 1 cup of the cheese and season with pepper; set aside.
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11
Grate the potatoes on the large holes of a box grater (you should have about 5 cups of shredded potatoes), add to the pot, and stir to combine.
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12
Chop the cooled bacon into small dice, add it to the potato mixture, and stir to combine.
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13
Transfer the mixture to the prepared baking dish, spread it into an even layer, and sprinkle with the remaining 1 1/2 cups of cheese.
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14
Bake until browned all over, bubbling around the edges, and the potatoes are cooked through, about 1 hour (cover the dish with aluminum foil if its browning too quickly).
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15
To make ahead and reheat in an oven or on a grill, let the cooked casserole cool to room temperature.
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16
Cover with aluminum foil and refrigerate for up to 2 days.
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17
When ready to reheat, let the covered casserole sit at room temperature for 1 to 1 1/2 hours, then place in a 350 degrees F oven or on a medium grill (about 350 degrees F to 450 degrees F) until heated through, about 20 to 30 minutes.
