The panino con ricotta romana is built on a fresh, delicate cheese and the discipline that delicacy demands. Ricotta Romana DOP is made from the whey of sheep's milk from the Lazio flocks, gently re-cooked so the fine curds rise: white, soft, faintly grainy, with a clean sweet-savoury milkiness and a perishable freshness that is the whole character of the thing. This is not the pressed, dried ricotta that gets grated over pasta; it is a spoonable curd that lives or dies on how recently it was made. The sandwich is therefore an exercise in not getting in its way, since anything sharp or heavy beside it would erase a cheese whose appeal is its mildness.
The craft is handling something fragile so it still reads. The ricotta is spread thick rather than thin, because a delicate cheese laid sparingly disappears, and it is taken barely cool so the curd is loose and sweet rather than tight and bland. It weeps if it sits, so the bread is plain and soft and the cheese goes in close to eating; a sturdy or assertive loaf would both flood and overpower it. The classic counter is a single quiet accent that pulls the sweetness one way or the other: a thread of honey and a grind of pepper for the sweet-savoury reading, a little salt and good oil for the savoury one, occasionally a few greens for a bitter edge. The point is one fresh curd, fresh, and one small decision about which way to tilt it.
The variations follow that single tilt, each its own preparation rather than a footnote here: the sweet build with honey and pepper; the savoury one with oil, salt, and pepper; and the ricotta e spinaci version where wilted greens are folded in. Each is the same fresh-sheep-ricotta logic given a loaf, and each deserves its own article rather than being crowded in here.