1 large onion
2 lb russet potatoes (2 large)
2 1/2 Tbsp butter
2 Tbsp flour
1/8 tsp dry (ground) mustard
1 1/2 cups milk
1/4 tsp salt
pepper to taste
4 oz (1 cups) grated sharp Cheddar
1/4 cup bacon bits
1 sliced scallion
Turn the oven on to 375. Spray a casserole dish or pan with baking spray or butter it generously. Peel the onion and potatoes. Chop the onions and slice the potatoes into 1/8 - 1/4” slices.
Put 1 cup of water in the Instant Pot. Toss the potatoes and onion to combine evenly and put them in a steamer basket in the IP. Close the lid and set the valve to Sealing. Push Pressure Cook (or Manual) and adjust time using the + and - buttons to get to one minute.
While the potatoes and onions are in the pot, make the cream sauce (Béchamel). Heat the milk in the microwave. Melt the butter in a saucepan over medium-high heat. Whisk the ground mustard and flour into the melted butter until smooth. Cook and stir for a minute or so until bubbly. Whisk in the hot milk all at once and bring to a boil. When it has thickened, remove from heat and add salt and pepper. **Important: be generous with the salt and pepper; potatoes absorb a lot of seasoning.
When the Instant Pot beeps that it's done, flip the valve to Venting for a quick release. When the pin drops, remove the potatoes and onions.
Pour about half the potatoes and onions into the prepared casserole dish. Give the dish a shake to settle them a bit. Pour about half the sauce over them. Shake the dish again to distribute the sauce evenly. If you are adding cheese, bacon or scallions, add most of them now, saving some for the top. Add the rest of the potatoes and onions and cover with the remaining sauce. Shake it again to get that sauce down in between the potatoes.
Sprinkle with the remaining cheese and bacon over the top, if using, and put it into the oven for 15 - 20 minutes or until golden brown.