Bring a large pot of salted water to a boil. Once boiling, add spaghetti. Cook, stirring occasionally, until al dente, 9-11 minutes. Reserve ½ cup pasta cooking water (1 cup for 4 servings), then drain.
Meanwhile, wash and dry all produce. Trim and halve zucchini lengthwise; cut crosswise into ¼-inchthick half-moons. Dice tomato; toss in a small bowl with a drizzle of olive oil and a pinch of salt and pepper. Thinly slice half the chili (if you like spicy food, slice the whole thing). Quarter lemon. Remove sausage from casing; discard casing.
Heat a large drizzle of olive oil in a large pan over medium-high heat. Add zucchini in a single layer; season with salt and pepper. Cook, stirring occasionally, until browned and tender, 5-6 minutes. Transfer to a plate.
Heat a drizzle of olive oil in same pan over medium-high heat. Add sausage and cook, breaking up meat into pieces, until browned and cooked through, 4-6 minutes. (TIP: If there’s excess grease in your pan, carefully pour it out.) Stir in tomato and a pinch of chili to taste. Cook until slightly softened, 1 minute.
Add spaghetti, garlic herb butter, half the Parmesan (save the rest for serving), ¼ cup reserved pasta cooking water (⅓ cup for 4 servings), and 2 tsp lemon juice (1 TBSP for 4). Toss until thoroughly combined. (TIP: If needed, add more pasta cooking water a splash at a time until pasta is coated in sauce.) Stir in half the zucchini and season with plenty of salt and pepper.
Divide pasta between bowls. Top with remaining zucchini and Parmesan. Garnish with a pinch of remaining chili if desired. Serve with remaining lemon wedges on the side.