This recipe is the Roman version of Mac 'n Cheese - fresher, lighter and well... Italian! Even though you add the "sauce" after pressure cooking, this recipe takes advantage of the boiling pasta water to melt the cheese (a step that would ordinarily be done separately with the pasta cooking water).
Original versions of this dish only have Percorino Romano cheese and pepper.. lots of it! So much that the result is very spicy (like many dishes from this region). But the last couple of generations of Roman housewives have upgraded this dish without making it any more complicated with the addition of Ricotta cheese and a dash of fresh herbs.
Since Ricotta is such a dominant flavor in this dish... buy the good stuff or make it yourself from a gallon of whole milk!