Add 1 tablespoon unsalted butter and 1 tablespoon olive oil to large skillet and place skillet on stovetop over medium-high heat. Heat skillet until butter is melted and oil is shimmery, swirling and tilting pan occasionally to distribute oil and butter across entire surface.
Reduce heat under skillet to medium. Add 1 cup chopped yellow onion and ½ cup julienned carrots to skillet and stir to incorporate. Sauté onions and carrots approximately 5 minutes, stirring occasionally, or until aromatics have softened.
Once onions and carrots have softened, add 2 cups chopped broccoli florets, 8 ounces sliced mushrooms, 1 ½ cups chopped zucchini, and 2 teaspoons minced garlic to skillet. Top vegetables with 1 tablespoon olive oil and stir gently to incorporate, covering all vegetables in oil and remaining melted butter.
Season vegetables with 1 tablespoon Italian seasoning, salt to taste, and freshly cracked black pepper to taste. Stir gently to coat all vegetables. Sauté vegetables 5 minutes, uncovered, stirring often.
While vegetables sauté, fill large pot with 16 cups water and place pot on stovetop over medium-high heat. Bring pot of water to rolling boil, then add 1 tablespoon salt and stir to incorporate.
When water begins to boil again, add 1 pound uncooked capellini to pot and cook according to package instructions. Reduce cook time on packaging 1 minute.
When pasta is al dente, pour pasta and water into colander and drain pasta well. Set aside.
Add ¼ cup sun-dried tomatoes in oil, 1 cup crushed tomatoes, and 1 ½ cups low-sodium vegetable broth. If desired, also add ¼ teaspoon crushed red pepper flakes. Gently stir all ingredients together until well incorporated, then increase heat under skillet slightly and bring liquid to gentle boil.
When liquid begins to boil, reduce heat under skillet to medium-low and cover skillet with lid. Simmer mixture 7 to 10 minutes, stirring occasionally, until vegetables are tender.
Divide cooked pasta into desired portions and plate. Top plated pasta with preferred amount of primavera sauce, then garnish with grated parmesan and chopped fresh basil (optional). Serve warm with garlic-buttered breadsticks if desired.