Cook pasta to your liking and drain.
In a cast iron skillet, melt butter.
Sprinkle on flour and whisk 2 minutes to cook.
Slowly add milk/half and half, whisking to combine.
Add mustard and salt.
Whisk about 5 minutes, it should get bubbly and thicken up a bit.
Add in cheeses and stir to melt them.
Once smooth, add pasta and sprinkle on bacon.
Serve immediately!