Bring a large pot of salted water to a boil. Wash and dry all produce. Trim and quarter zucchini lengthwise; cut crosswise into ½-inch-thick pieces. Mince garlic. Zest and quarter lemon (for 4 servings, zest 1 lemon and quarter both).

Once water is boiling, add spaghetti to pot. Cook until al dente, 9-11 minutes. Reserve 1 cup pasta cooking water, then drain. Keep empty pot handy for use in step 5.

Meanwhile, heat a drizzle of olive oil in a large, preferably nonstick, pan over medium-high heat. Add zucchini; cook, stirring occasionally, until browned and softened, 4-6 minutes. Season with salt and pepper. Turn off heat; transfer to a plate. Wipe out pan.

While zucchini cooks, pat chicken dry with paper towels and season all over with Italian Seasoning, salt, and pepper. Once zucchini is done, heat a large drizzle of oil in same pan over medium-high heat. Add chicken and cook until browned and cooked through, 5-7 minutes per side. Turn off heat; transfer to a cutting board to rest. Once cool enough to handle, slice chicken crosswise.

Heat a drizzle of olive oil in pot used for spaghetti over medium-high heat. Add garlic, half the lemon zest, and a pinch of chili flakes. Cook, stirring, until fragrant, 30 seconds. Stir in ½ cup reserved pasta cooking water (¾ cup for 4 servings), stock concentrate, and juice from half the lemon. Simmer until thickened, 1-2 minutes. Turn off heat.

Add spaghetti, zucchini, sour cream, and 1 TBSP butter (2 TBSP for 4 servings) to sauce; toss to coat. Add half the Parmesan; season with salt and pepper. (TIP: If needed, stir in more reserved cooking water a splash at a time until pasta is thoroughly coated in sauce.) Divide between bowls and top with chicken. Top with remaining Parmesan and lemon zest, and a pinch of chili flakes if desired. Serve with remaining lemon wedges on the side.