Bring a large pot of well-salted water to a rolling boil. Add the pasta and cook according to package directions until al dente, stirring occasionally to prevent sticking. Reserve about ½ cup of the pasta water before draining. Drain the pasta and set aside.

While pasta cooks, wash and slice all vegetables into similar-sized pieces. Slice harder vegetables like carrots and broccoli slightly thinner than softer ones like zucchini for even cooking.

Heat olive oil in a large skillet over medium heat. Add red onion and carrots, cooking for 2-3 minutes until they begin to soften. Stir in broccoli and bell peppers, cooking for another 3-4 minutes until tender-crisp and vibrant. Add zucchini last and cook for 1-2 minutes until slightly softened, ensuring not to overcrowd the pan.

Add minced garlic, Italian seasoning, salt, black pepper, and red pepper flakes (if using) to the skillet. Stir constantly for about 30 seconds until fragrant, being careful not to burn the garlic.

Add the cooked pasta directly into the skillet with the sautéed vegetables. Toss gently to combine, ensuring the pasta is coated. Add a splash of the reserved pasta water if the mixture appears dry, which helps create a light, silky sauce.

Remove the skillet from heat. Stir in fresh lemon juice and grated Parmesan cheese. Taste and adjust seasoning as needed. Garnish with fresh basil or parsley just before serving.