The panino con fregola is carb on carb, and that frank oddity is what defines it. Fregola is a Sardinian semolina pasta rolled into small irregular pearls and toasted in the oven until they take on a nutty colour and a firm, almost gritty bite. It is a pasta, usually served in broth or with clams, and putting it inside bread is an unusual move that the sandwich does not pretend otherwise. The filling is essentially a cold fregola salad, dressed and bound, asked to behave between two pieces of a loaf that is already starch.
The craft is making loose toasted pearls hold together in bread without the whole thing reading as a heavy double dose of the same thing. The fregola is cooked, cooled, and dressed so the pearls stay separate but coated, bound just enough with oil and the moisture of its other components, often tomato, herbs, a little fish or vegetable, so it does not spill out of the bread in a dry scatter. The toasting of the semolina is what earns its place: it gives the pearls a roasted depth and a chew that a plain pasta would not bring, which is the only reason a starch inside a starch is interesting rather than redundant. The bread is plain and sturdy, doing as little as possible, since the point is the fregola and an assertive loaf would just compound the weight. It is dressed lightly and eaten without delay, before the bread takes up the dressing.
The variations follow the Sardinian table the fregola comes from: the version dressed with tomato and herbs, the one carrying a little preserved fish, the lighter vegetable build. Each is a different fregola preparation pressed into bread, and each deserves its own article rather than being crowded in here.