Bring a large pot of generously salted water to a boil. Cook the refrigerated tortellini according to the package directions, usually 3 to 5 minutes, then reserve 1/2 cup cooking water and drain.

Heat the olive oil in a large skillet over medium-high heat. Add the onion, zucchini, bell pepper, and broccoli, and cook for 5 to 7 minutes, stirring occasionally, until the vegetables begin to soften.

Add the garlic and cherry tomatoes to the skillet. Cook for 1 minute, just until the garlic is fragrant and the tomatoes begin to warm.

Push the vegetables to one side of the skillet and melt the butter in the open space. Whisk in the flour and cook for about 1 minute to form a smooth roux.

Gradually whisk the whole milk and heavy cream into the flour mixture. Add the Italian seasoning, kosher salt, and black pepper, then simmer for 3 to 5 minutes, stirring frequently, until thickened.

Stir the finely grated Parmesan into the hot sauce until melted and smooth. Keep the heat moderate so the sauce stays creamy rather than boiling aggressively.

Add the drained tortellini and baby spinach to the skillet. Toss gently for 1 to 2 minutes, until the spinach wilts and the pasta is fully coated; add reserved pasta water a tablespoon at a time if needed.

Finish with chopped fresh basil or parsley and additional Parmesan if desired. Serve immediately while the sauce is glossy and warm.