That recipe is indeed Béchamel sauce (with mustard), but I use half the amount of butter.
There is a trick to make Béchamel sauce without the lumps. Heat the butter, toss the flour and let it roast to a golden colour. Then add a bit of milk to dilute the paste and then add the whole rest of the milk and stir vigorously, so that the flour is spread into the whole volume of milk.
At that point, you have an emulsion of flour in milk and you want to cook it so that the flour sets and gives a thick sauce. So you turn down the heat and stir all the time till the sauce thickens. You stir so that you get a constant temperature over the whole volume. If you don't stir, the bottom will overheat, cook earlier and that layer of overcooked sauce will give the lumps.
If you miss and get lumps, use a blender...
I would also not add the grated cheese to the Béchamel sauce, that is not likely to work. I would mix the pasta and the grated cheese and then add the Béchamel sauce. The cheese will melt in the oven.