Tofu Subs with Onions and Peppers

Name: Tofu Subs with Onions and Peppers
Ingredients: 1 small onion, halved and thinly sliced; 2 bell peppers (1 red, 1 green), thinly sliced; 3 tablespoons extra-virgin olive oil; Kosher salt; 1/2 teaspoon red pepper flakes; 1 teaspoon fennel seeds, crushed; 10 ounces extra-firm tofu, sliced into 8 pieces and patted dry; 4 whole-wheat sub rolls, split; 1/3 cup marinara sauce, warmed; 2 tablespoons balsamic vinegar; Freshly ground pepper; 6 cups baby arugula; 1 ounce parmesan cheese, shaved with a vegetable peeler