Heat olive oil in a small saucepan over medium heat.

Add chopped onion and minced garlic.

Cook, stirring occasionally, until the onion is tender, which should take about 3 to 4 minutes.

Stir in the cauliflower florets and vegetable broth.

Bring the mixture to a boil, then reduce the heat and simmer, covered, until the cauliflower is tender, approximately 15 minutes.

Uncover and allow the mixture to cool slightly.

Transfer the cooked cauliflower to a food processor.

Add the shredded Parmesan cheese, water, and black pepper.

Cover and pulse until the mixture is smooth.

Meanwhile, cook the multigrain spaghetti according to the package directions, adding the broccoli and bell pepper during the last 5 minutes of cooking.

Drain the pasta and vegetables, reserving 1/2 cup of the cooking water.

Return the pasta and vegetable mixture to the pot and stir in the cauliflower sauce.

Heat the mixture through, and stir in reserved cooking water (if needed) to reach your desired consistency.

Garnish the dish with basil, lemon zest (if using), lemon juice, and additional Parmesan cheese.

Enjoy your Pasta Primavera with a creamy cauliflower sauce!