Adjust rack to top position and preheat oven to 450 degrees. Wash and dry all produce. Trim and halve zucchini lengthwise; scoop out and discard seeds. Rub all over with a drizzle of olive oil, season with salt and pepper. Arrange cut sides down on a baking sheet. Roast on top rack until browned and tender, 15-20 minutes.

Meanwhile, heat a drizzle of olive oil in a large, preferably oven-safe, pan over medium-high heat. Add sausage and Tuscan Heat Spice. Cook, breaking up meat into pieces, until browned all over, 4-5 minutes (the sausage will finish cooking in the next step).

Stir couscous, tomato paste, and 11/2 cups water (21/2 cups for 4 servings) into pan until combined. Bring to a simmer and cook until mixture has thickened, couscous is tender, and sausage is cooked through, 6-8 minutes. (TIP: If pan seems dry, add a splash of water.) Season generously with salt and pepper. Turn off heat.

Meanwhile, dice tomato. Pick basil leaves from stems; discard stems and roughly chop leaves. In a small bowl, combine tomato, half the basil, and a large drizzle of olive oil. Season with salt and pepper. Stir remaining basil into couscous mixture.

Remove zucchini from oven. Flip cut-sides-up, then stuff with as much couscous mixture as will fit. Nestle on top of remaining couscous mixture in pan. (TIP: If pan is not ovenproof, transfer everything to a baking dish at this point.) Sprinkle stuffed zucchini with cheese. Return to top rack until cheese has melted, 1-2 minutes.

Top stuffed zucchini with tomato-basil mixture. Serve straight from the skillet or divide between plates.