Meanwhile, in a large and deep skillet or saucepan, melt butter over medium heat. Add garlic, flour and onion powder and cook, stirring constantly, 1 minute.
While whisking, slowly pour in milk and cream, then whisk until smooth.
Cook, stirring constantly, until mixture begins barely simmer and thicken, then add tomatoes, spinach, basil, oregano and parmesan. Season with salt and pepper to taste.
Reduce heat slightly and cook several minutes longer until sauce has thickened and cheese has melted. Remove from heat.
Toss cooked and drained tortellini into sauce mixture, thin sauce with a little pasta water or milk as needed.
Serve warm sprinkled with additional parmesan cheese and optional red pepper flakes.