Cook the Tortellini: In a large pot, bring salted water to a boil. Add the cheese tortellini and cook according to package instructions until al dente. Drain and set aside.

Prepare the Sauce: In a large skillet, melt the butter over medium heat. Add the minced garlic and cook until fragrant, about 1 minute.

Make the Roux: Add the flour and onion powder to the skillet, whisking constantly to form a smooth paste. Cook for 1–2 minutes to eliminate the raw flour taste.

Add Liquids: Slowly pour in the milk and heavy cream, whisking continuously to prevent lumps. Bring the mixture to a simmer and cook until it thickens slightly, about 3–5 minutes.

Incorporate Vegetables: Stir in the diced tomatoes (with their juices), chopped spinach, basil, and oregano. Cook until the spinach is wilted and the mixture is heated through, about 2–3 minutes.

Combine with Tortellini: Add the cooked tortellini to the skillet, tossing gently to coat in the sauce. Stir in the grated Parmesan cheese until melted and the sauce is creamy. Season with salt and pepper to taste.

Serve: Ladle the creamy tortellini into bowls. Sprinkle with additional Parmesan cheese and crushed red pepper flakes, if desired. Serve immediately.