Bring a large pot of water to a vigorous boil. Add 1 tablespoon salt. Add 12 oz spaghetti and cook according to package until al dente (usually 8–10 minutes). Reserve 1 cup of pasta cooking water, then drain the pasta and set aside.

Vegetables

Prep vegetables

While the water heats, trim and slice vegetables: halve mushrooms (or slice thickly), cut zucchini into 1/2-inch half-moons, trim tough ends from asparagus and cut into 1–1.5 inch pieces (tips reserved whole if desired), trim broccolini stems and cut longer stems in half lengthwise if thick, peel and thinly slice the onion, and mince 4 garlic cloves.

Sauté vegetables

Heat a large wide skillet or sauté pan over medium-high heat. Add 2 tablespoons olive oil. When oil shimmers, add sliced onion and a pinch of salt; cook, stirring occasionally, until softened and just beginning to brown, about 4 minutes. Add mushrooms and cook 3–4 minutes until they release moisture and begin to brown. Push the vegetables to one side, add remaining 1 tablespoon olive oil if pan looks dry, then add zucchini and asparagus pieces. Sauté 3–4 minutes until zucchini is tender-crisp. Add broccolini and cook another 2–3 minutes until bright green and just tender. Stir in minced garlic, red pepper flakes (1 tsp), dried oregano and dried basil; cook 30–45 seconds until fragrant. Season vegetables with 1/2 teaspoon of the salt and 1/2 teaspoon freshly ground black pepper.

Sauce & Combine

Add marinara

Pour 2 cups marinara sauce into the skillet with the vegetables and stir to combine. Bring to a gentle simmer and let cook 3–5 minutes so flavors meld. Taste and add remaining 1/2 tsp salt and remaining black pepper if needed. If the sauce is too thick, add up to 1/2 cup reserved pasta water to loosen.

Finish pasta with sauce

Add drained spaghetti directly to the skillet (or return spaghetti to the pot and pour sauce over). Toss gently to coat, adding reserved pasta water in 1/4 cup increments as needed to achieve a silky sauce that clings to the pasta (use up to the full 1 cup reserved). Cook together over low heat 1–2 minutes so the pasta absorbs some sauce.

Finish and serve

Remove from heat. Stir in 1 tablespoon lemon juice (optional) and 2 tablespoons chopped fresh parsley. Taste and adjust seasoning. Serve immediately with grated Parmesan or Pecorino at the table.