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 pasta 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, about 1 minute to form a roux.
Slowly whisk in the milk and heavy cream until smooth, then cook, stirring, until the sauce just begins to simmer and thickens slightly.
Stir in the undrained diced tomatoes, chopped spinach, chopped basil, chopped oregano, and the shredded Parmesan; season with salt and freshly ground black pepper to taste.
Reduce the heat slightly and cook a few more minutes until the sauce has thickened and the cheese has melted, removing from heat when done.
Toss the drained tortellini into the sauce, thinning with reserved pasta water or a splash of milk if needed to coat evenly.
Serve warm sprinkled with additional Parmesan and optional red pepper flakes.