Begin by adjusting your oven's rack to the upper third position and preheat it to 425°F. Slice each zucchini in half lengthwise. Using a spoon, remove and discard the seeds. Further, scoop out the pulpy center to form shallow zucchini "shells" with approximately 1/2 inch thick walls. Roughly chop the scooped-out zucchini flesh and set aside for later use. Arrange these zucchini shells in a baking dish.

In a skillet, heat one tablespoon of the olive oil over a medium-high flame. Add the turkey sausage, chopped red onion, and diced bell pepper to the pan. Use a wooden spoon or a heatproof spatula to crumble the sausage while it cooks with the vegetables. Continue to sauté until the sausage is thoroughly cooked and browned, and the onions have softened, about 5 to 8 minutes. Season the mixture with salt, black pepper, and red pepper flakes.

Stir in the minced garlic and tomato paste, cooking for another minute until the paste is well integrated. Incorporate the reserved chopped zucchini flesh, letting it warm for about a minute before removing the skillet from the heat. Adjust seasoning with additional salt, pepper, or red pepper flakes to suit your taste.

Meanwhile, toast the bread slices until golden. Tear them into larger pieces and place them in a food processor equipped with a steel blade. Add the fresh basil leaves and the remaining tablespoon of olive oil. Pulse the mixture until the bread turns into coarse crumbs. Introduce the grated Parmesan to the processor and pulse a few times just to mix.

Fold half of the basil breadcrumb mixture into the sautéed sausage. Use a small spoon to generously fill the zucchini shells with the stuffing mixture. Sprinkle the remaining breadcrumbs over the stuffed zucchini. Transfer the baking dish to the oven and roast until the zucchini is tender and the breadcrumbs have turned a delightful golden brown, which should take about 13 to 15 minutes. Serve the stuffed zucchini warm, sprinkled with additional Parmesan if desired.