Bring a large pot of salted water to a boil and cook the refrigerated tortellini according to package directions; reserve about 1/2 cup pasta cooking water, then drain.
While the tortellini cooks, melt the butter in a large deep skillet or saucepan over medium heat.
Add the minced garlic, flour, and onion powder to the melted butter and cook, stirring constantly, for 1 minute to form a roux.
Slowly whisk in the milk and heavy cream until smooth and continue cooking, stirring frequently, until the mixture just begins to simmer and thickens slightly.
Stir in the undrained diced tomatoes, chopped spinach, chopped basil, oregano, and the shredded parmesan; season with salt and freshly ground black pepper to taste.
Reduce the heat a little and cook for a few minutes until the sauce has thickened and the cheese has melted.
Add the drained tortellini to the sauce and toss to coat, thinning the sauce with a little reserved pasta water or additional milk if needed to reach your desired consistency.
Serve warm sprinkled with extra parmesan and optional red pepper flakes.