In the frying pans you used for the meatballs (don't clean them, there is flavour in the bits and you're going to use that to your advantage now) you are going to cook the onions and shallots in olive oil until they caramelize. This will take about 10 or 12 minutes depending on heat, dice size and your olive oil. You want to cook them over a medium heat until they turn a golden colour.
Now add the barbecue sauce, water and garlic to the pans.
Bring that to a hard boil. Allow it to continue to boil for about 2 or 3 minutes. It will start to thicken just a touch. Now pour that over top off your meatballs you set aside. Cove the meatballs, and pop them in the preheated oven for 30 minutes.
After 30 minutes you will remove the cover and continue to cook them for about 10 to 15 minutes longer. Baste them halfway through.
Remove the meatballs from the oven. Now turn the oven up to 400 degrees F and allow it to hit that temperature before popping grinders into the oven. You need it hotter so it melts the cheese nicely.
Now scoop out the meatballs with a wire scoop so you get the meatballs but not the sauce at this point. Do NOT discard the barbecue mixture. Set it aside. You're going to use this.
I cut the meatballs in half but you don't have to. I just find they sit on the grinder buns better that way. Use your own judgement.
Putting the grinders together:
Your oven should be preheating to 400 degrees F.
Your two rimmed cookie sheets should be lined with parchment.
Your barbecue sauce from the meatballs should be nearby for drizzling.
Your meatballs should be sliced in half, or not, as the case may be.
Okay now that you've checked off your to do list you can start assembling.
Slice your buns BUT NOT all the way through. Leave the top and bottom attached and stop about a ¼ inch from the end. Makes them easier to fold over and not lose ingredients because they are still attached.
Lay the buns on the parchment lined cookie sheets.
Lay down a layer of mozzarella cheese slices on the bottom part of each bun.
Now place rows of meatballs down each bun.
Take a slotted spoon and scoop out the onions and shallots with a little barbecue juice as well onto a plate, or in a bowl.
Now take a spoon and spoon a bunch of that oniony/ shalloty/bbqy goodness over each sub. Use as much or as little as you like. I used every last onion and shallot bit on mine, I then discarding the leftover sauce since it's pretty fatty at this point.
Now take the fresh ¼ cup of barbecue sauce and pour a little over each one.
Top with as much cheese as you can fit on each one.
Leave the grinders sort of face up on the sheets.
Pop them in the preheated oven for about 8 to 10 minutes.
Remove from oven.
Serve with a big old "omg I need a napkin in each hand" smile!