Meatball Sliders  1 red onion cut into thin wedges (1 1/2 C)  2  12 oz package frozen cooked meatballs (24)  1  24 - 26 oz jar marinara or pasta sauce (about 2 1/4 cups)  1  T balsamic vinegar  1/2  t crushed red pepper  6  slices provolone, quartered  4  roma tomatoes, sliced   24 cocktail buns,