You can eat them as-is, the way I presented them in my Italian-American meatball piece. You could toss the sauce with spaghetti and set the meatballs on top. You could follow Kenji's approach and make smaller meatballs that are a perfect topper for homemade pizza. Or you could construct one hell of a meatball sandwich.
With the meatballs and sauce already figured out, a sandwich is really just a matter of construction.
First, pick your bread. Personally, I like an Italian-style roll, one of those loaves that's a little wider than a traditional French baguette, as white as bleached linens, super soft throughout, with a potato-chip-thin crust that crackles and makes a mess of crumbs and flakes on your shirt when you bite through it. My reasoning is that meatballs are (or should be) tender, and I don't want my bread fighting with them—a loaf that's too sturdy won't play nice with such a squishable filling. The crust should have just enough personality to add some crackle, but nothing substantial enough to make biting difficult.
I warm the loaf in an oven, but don't toast for the same reason I choose soft bread: except for that flaky crisp crust, I want tenderness to surround my meatballs.* Once it's warmed, I trim off the knobby, tough ends of the loaf, because, honestly, does anyone really like gnawing through those parts? I know I don't, and I especially don't like the way crusty ends can crush a tender meatballs. Then I slice the bread in half, drizzle some olive oil on the top and bottom pieces, and give them a quick rub with a raw clove of garlic to punch the flavor up a little bit.
*It took so much self control to not make a crude joke there.
Next, I spoon a layer of sauce onto the bottom half of the loaf. Some folks like to toast their bread before spreading on the sauce and meatballs. Sure, it keeps them tidier, but for me, that defeats the whole point of a sauced sandwich. It's supposed to have some of that soft, wet texture. Don't get me wrong: I don't want a soggy sandwich, but a little bit of sauce-soaking is a good thing.
For the meatballs themselves, I like to make them large and then slice them in half. Small ones will fall out of the sandwich too easily, while whole large ones are difficult to get your mouth around—and they don't cozy up next to each other well. Halved meatballs still have a good height, while their tapered edges are able to overlap slightly for even coverage. This way you get a good amount of meatball in each bite. I realize cutting the balls in half means they're no longer technically meatballs, but for me, the dome shape is enough to still suggest the experience of a meatball, and the benefits are worth it. I can live with whatever semantic discord this might cause.
Then I spoon more sauce on top of the meatballs.
A generous shower of Parmesan adds a salty, savory touch.
I lay slices of mozzarella on top. I prefer sliced mozzarella to grated mozzarella for its even, bedsheet-like coverage.
You can use fresh mozzarella or low-moisture mozz to get different results. Fresh mozzarella can be a little bit too wet, its moisture pooling up or turning the bread soggy. Low-moisture mozzarella is gooey when really hot, but can turn a little rubbery as it cools. The stuff you see here is the kind that's sold at the supermarket labeled "fresh" though really, it isn't (we're talking the stuff stored in sealed cryo-vacked bags, not in tubs water or brine). In effect, it's somewhere between the two, neither as wet as the real fresh stuff, nor as rubbery as the low-moisture kind. I think it hits a good compromise.
I set the whole thing in the oven just long enough to melt the cheese, reheating the top half of the bread for the last minute of cooking.
Close the sandwich and you're in business.
Just look at that: I was able to slice it in to little portions to share without the meatballs squishing out, and each piece is as desirable as the next.
The only trick is pulling all this off without eating the meatballs by themselves first.