Combine cipollini onions, vinegar, and salt in a food processor or blender; puree until nearly smooth. Set aside for 1 hour.
Heat olive oil in a large skillet over medium-high heat; cook and stir onion, red bell pepper, and goat horn peppers in the hot oil until onion softens and begins to turn golden, 7 to 8 minutes. Remove onion mixture to a plate. Cook and stir coppa ham, soppressata, pork loin, and mortadella in the same skillet until warmed through and lightly browned, about 5 minutes.
Spread each ciabatta roll with 1 tablespoon garlic basil spread, 1 tablespoon garlic roasted tomato sauce, 1 tablespoon artichoke aioli, and 1 tablespoon cipollini onion mixture. Top with 1/4 pound mozzarella cheese. Evenly distribute the soppressata mixture among the 4 sandwiches and top with equal amounts of the bell pepper mixture.
Arrange sandwiches on a baking sheet and bake in the preheated oven until the cheese is melted and the bread is toasted, 3 to 4 minutes.
The cipollini onion mixture can be made up to 1 month in advance and stored in the refrigerator.