Pasta Primavera with Cauliflower Sauce

Heat olive oil in a small saucepan over medium heat. Add onion and garlic; cook, stirring occasionally, until onion is tender, 3 to 4 minutes. Stir in cauliflower and broth; bring to a boil. Reduce heat and simmer, covered, until tender, about 15 minutes. Uncover and let cool slightly.

Meanwhile, bring a large pot of salted water to a boil. Cook spaghetti in boiling water until tender yet firm to the bite, 7 to 10 minutes, adding broccoli and bell pepper for the last 5 minutes. Drain, reserving 1/2 cup cooking water. Return spaghetti to the pot and set aside.

Transfer cauliflower-broth mixture to a food processor. Add Parmesan cheese, water, and black pepper. Cover and pulse until sauce is smooth.