Set the Instant Pot to saute then add the butter and allow it to melt. Sprinkle in the flour and stir to create a roux.
Stir in the salt, pepper, ground mustard, milk, and heavy cream.
Once the cream mixture is hot, slowly add in the cheese, one handful at a time to prevent clumping. Stir continuously until the cheese is incorporated.
Add the cooked pasta back into the cheese sauce that you have created stir until all of the pasta is coated.