Wash and dry all produce. Bring a large pot of salted water to boil. Trim ends from zucchini, then quarter leghtwise. Thinly slice quarters crosswise into triangles. Halve, peel, and thinly slice onion. Mince or grate garlic. Cut lemon into wedges. Remove sausage from casing; discard casing.
Heat a large drizzle of olive oil in a large pan over medium-high heat. Add sausage and cook, breaking meat up into pieces, until browned and cooked through, 5-7 minutes. Season with salt and pepper. Remove from pan and set aside.
Once water boils, add linguine to pot and cook, stirring occasionally, until al dente, 9-11 minutes. Scoop out and reserves 1 cup pasta cooking water, then drain. Return linguine to pot along with a splash of cooking water.
Add onion, zucchini, and another large drizzle of olive oil to pan used for sausage over medium-high heat. Cook, tossing, until softened, about 4-6 minutes. Toss in garlic, thyme and chili flakes (to taste). Cook until fragrant, about 1 minute. Season with salt and pepper.
Add diced tomatoes and sausage to pan with veggies and stir to combine. Season to taste with salt and pepper. Let simmer until rest of meal is ready.
TIP: Give the sauce a taste. If it seems sharp, add up to 1 tsp sugar.
PLATE AND SERVE
Add sausage, veggies, and sauce to pot with linguine. Add a squeeze or two of lemon (to taste). Toss to combine. Season with salt and pepper. (TIP: If sauce seems thick or stiff, stir in some pasta cooking water.) Divide mixture between plates and sprinkle with Parmesan. Serve lemon wedges on the side for squeezing over.