Preheat the oven to 450 degrees F. Place the onion, zucchini, red pepper, eggplant and garlic in a bowl. Add the olive oil, balsamic vinegar, salt and pepper. Place in a single layer on a parchment paper-lined baking sheet. Roast the vegetables for 20 to 25 minutes, or until nicely roasted and just tender. Remove the vegetables from the oven and sprinkle with the basil or parsley. Slice the croissants in half and stuff them with the lettuce, roasted vegetables and slices of the cheese.