Cook the tortellini according to package instructions until al dente, drain well, and set aside.
In a large saucepan over medium heat, melt the butter until foaming.
Whisk in the flour and cook for about 30 seconds, then add the minced garlic and onion powder and cook, stirring, for another 30 seconds.
Slowly pour in the milk and heavy cream while whisking to make a smooth sauce.
Continue cooking and stirring constantly until the sauce begins to simmer and slightly thickens.
Add the diced tomatoes (with juices), chopped spinach, chopped basil, and dried oregano to the sauce, then taste and season with salt and freshly ground black pepper as needed.
Stir in the grated Parmesan until melted and the sauce is smooth.
Remove the pan from the heat and fold the cooked tortellini into the sauce until well coated.
Serve warm with extra Parmesan and crushed red pepper flakes on top, if desired.
