1/2cupfreshly grated Parmesanplus additional for serving
Place a rack in the upper third of your oven. Preheat to 425°F.
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.
Fold half of the basil breadcrumbs into the sausage mixture. With a small spoon, mound the sausage stuffing into the shells, then sprinkle the tops of the stuffed zucchini with the remaining breadcrumbs. Roast in oven until the zucchini is heated through and the breadcrumbs are golden, 13 to 15 minutes. Enjoy immediately.
TO STORE: Store leftovers in an airtight storage container in the refrigerator for up to 3 days.
TO REHEAT: Rewarm stuffed zucchini in a casserole dish the oven at 350 degrees F until warmed through. If the breadcrumbs start to brown too much, loosely tent the dish with foil. You can also reheat this recipe in the microwave.
TIP: Grate cheese the easy way! Before starting the recipe, grate an entire wedge of Parmesan cheese right in your food processor. Cut the wedge into chunks, place them in a food processor fitted with a steel blade, then pulse until ground. Measure out and set aside the 1/2 cup needed for the recipe. Store the remainder in your refrigerator for weeks. You'll always have fresh Parm on hand! Then proceed with making the stuffed zucchini, no need to wash the processor bowl in between.