Cook the refrigerated cheese tortellini according to package instructions; drain and set aside.
In a large saucepan over medium heat, melt the butter. Whisk in the flour to form a smooth roux and cook for about 30 seconds.
Add the minced garlic and onion powder to the roux and cook, stirring, for 30 seconds more.
Slowly pour in the milk and heavy cream while whisking, then continue whisking until the mixture is smooth.
Cook the sauce, stirring constantly, until it begins to simmer and slightly thicken.
Stir in the diced tomatoes (with juices), chopped spinach, chopped basil, and dried oregano; season with salt and freshly ground black pepper to taste.
Add the grated parmesan and stir until melted and the sauce is smooth.
Remove the sauce from the heat and fold in the cooked tortellini until well coated.
Serve warm with extra parmesan and crushed red pepper flakes, if desired.