Cook the tortellini according to package instructions in a large pot of boiling salted water. Drain and set aside.
In a large saucepan over medium heat, melt the butter. Whisk in the flour and cook for 30 seconds to form a roux.
Add the minced garlic and onion powder to the roux and cook, stirring, for about 30 seconds until fragrant.
Slowly pour in the milk and heavy cream while whisking constantly until the mixture is smooth.
Continue to cook, stirring frequently, until the sauce begins to simmer and slightly thickens.
Stir in the diced tomatoes, chopped spinach, chopped basil, and dried oregano. Season with salt and freshly ground black pepper to taste.
Add the grated Parmesan and stir until it melts into the sauce.
Remove the sauce from the heat and fold in the cooked tortellini until evenly coated.
Serve warm with extra Parmesan and crushed red pepper flakes if desired.