Halve the zucchini lengthwise, scoop out the seeds, and discard the seeds. Then, scoop out the soft center flesh to create shallow zucchini “shells,” with walls about 1/2 inch thick, reserving the center flesh. Arrange the zucchini shells in a baking dish. Roughly chop the center flesh and set it aside.

Heat 1 tablespoon olive oil in a skillet over medium-high heat, then add the sausage, onion, and bell pepper. Using a wooden spoon or heatproof spatula, break apart the sausage, sautéing it together with the vegetables, until the onions are soft and the sausage is browned and fully cooked, 5 to 8 minutes. Stir in the salt, pepper, and red pepper flakes.

Add the garlic and tomato paste and cook 1 additional minute, until tomato paste is broken down and incorporated. Add in the reserved zucchini flesh and heat through for 1 minute, then remove from the heat. Taste and add additional salt, pepper, or red pepper flakes as desired.

Toast the bread slices, then tear them into large pieces and place in the bowl of a food processor fitted with the steel blade. Add the basil and remaining tablespoon olive oil. Pulse until the bread is broken into large crumbs. Add the Parmesan and pulse just until incorporated, 2 or 3 times.