4 cups low-sodium vegetable broth
8 ounces linguine, uncooked
1 small onion, halved and sliced thin
1 pound broccoli crowns, cut into large florets (about 2 cups)
1 1/2 pounds asparagus, ends snapped off and cut into 2-inch pieces
4 ounces sliced white button mushrooms (about 1 cup)
4 large cloves garlic, minced
1/4 teaspoon crushed red pepper flakes (less for milder heat; more to increase heat)
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
2 tablespoons olive oil
3/4 cup frozen peas
1 small handful fresh parsley, chopped (about 1/2 cup chopped)
1/4 cup heavy whipping cream
2 tablespoons grated Parmesan cheese
2 teaspoons freshly grated lemon zest
Additional salt and pepper to taste
Additional Parmesan cheese for serving, if desired