This is a crucial moment for your pesto, as you need to season with salt and pepper. Keep in mind that Parmigiano and Pecorino are both dry and salty cheeses. You have to taste your sauce a few times in order not to go overboard when you are seasoning it. If the sauce is too thick, add a little more milk, if too runny, return to the heat and add a pat of butter mixed with an equal amount of all-purpose flour. The most important thing though is: besciamella should not taste floury. If you think your sauce is ready, but you can spot a hint of "flouriness" when you taste it, think again, and keep on cooking it for a few minutes more.