When combining cheeses, look for flavor and texture complementation. Pecorino Romano is a salty hard cheese (made from sheep’s milk) that adds just that extra bite without being overwhelming.   Because it’s a hard cheese, it's not the best melting cheese. That's why when using a hard cheese in a pasta al forno dish, finely grate the cheese so it won't leave any clumps in the sauce by not melting enough. You can substitute Pecorino Romano with Parmigiano Reggiano. Or you can mix the two.  Tip: I highly recommend not using pre-shredded cheese. Pre-shredded cheese comes with additives and preservatives that prevent the cheese from getting hard. Unfortunately, they also prevent the cheese from melting smoothly.  When it comes to melting cheese on top, fresh mozzarella cheese is my #1 choice. It melts deliciously, doesn't add any more saltiness, and will make these long gooey cheese strings between the plate and your mouth. A gooey, fresh mozzarella, perfectly baked, should be able to keep it together for longer than your arm can reach up.  Remember that perfect pizza you had. You take a bite and try to put the slice back on the plate, but the cheese won't disconnect between your mouth and slice. You start stretching the slice of pizza away from your body. You reach the end of your stretch and the cheese is still connected. That's good mozzarella. Don't forget that with mozzarella: you get what you pay for.  If you're not into mozzarella like we are, try Gouda, provolone, or fontina. Or experiment with something different.