Bring a large pot of salted water to a boil and cook the refrigerated tortellini according to package directions until al dente; reserve 1/2 cup pasta cooking water, then drain the tortellini.
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.
Cook, stirring frequently, until the sauce just begins to simmer and thickens slightly.
Stir in the undrained diced tomatoes, chopped spinach, chopped basil, oregano, and the 1/2 cup shredded Parmesan; season with salt and freshly ground black pepper to taste.
Reduce the heat a little and simmer for several minutes until the sauce is thickened and the cheese has melted.
Add the drained tortellini to the sauce and toss to combine, thinning the sauce with a splash of reserved pasta water or additional milk if needed to reach desired consistency.
Serve warm, sprinkled with extra Parmesan and optional red pepper flakes.
