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

Marinara Sauce Base

Heat 2 tablespoons olive oil in a large skillet or sauté pan over medium heat. Add the finely chopped onion and cook, stirring occasionally, until softened and translucent, about 5–6 minutes.

Add the minced garlic, 1 teaspoon crushed red pepper flakes, 1 teaspoon dried oregano, and 1 teaspoon dried basil. Cook, stirring, until fragrant, about 30 seconds to 1 minute, taking care not to burn the garlic.

Tomatoes & Vegetables

Stir in the tomato paste and cook 1 minute to caramelize slightly. Add the two 14 oz cans (28 oz total) diced tomatoes with their juices, bring to a gentle simmer, and reduce heat to maintain a low simmer.

Add sliced mushrooms to the simmering sauce. Cook, stirring occasionally, until mushrooms have released their liquid and softened, about 6–8 minutes.

Fold in the packed 4 cups baby spinach in batches, allowing each addition to wilt before adding more. Continue simmering the sauce for 6–8 minutes so flavors meld and sauce thickens slightly.

Finishing & Seasoning

Taste the sauce and add 1 teaspoon granulated sugar if it tastes too acidic. Season with 1 teaspoon salt and 1 teaspoon freshly ground black pepper (adjust to taste).

Add the drained linguine to the sauce and toss to combine. If sauce is too thick, add reserved pasta cooking water, a splash at a time, to loosen and create a glossy emulsion until the sauce coats the pasta (use up to the reserved 1 cup as needed).

Stir in torn fresh basil leaves (about 1 cup) if using. Remove from heat and taste; adjust seasoning.

Serve hot, topped with grated Parmesan cheese if desired.