Step 1: In a large pot, bring the vegetable or chicken broth to a boil. Add the elbow macaroni pasta and cook according to package directions until al dente. Drain the pasta, reserving the broth, and set both aside.
Step 2: In the same pot (or a separate large saucepan), melt the unsalted butter over medium heat. Once melted, sprinkle in the all-purpose flour, whisking constantly for 1-2 minutes to create a smooth roux. Cook until light golden, ensuring no raw flour taste.
Step 3: Gradually whisk in the milk to the roux until fully combined and smooth. Slowly pour in the reserved broth, continuing to whisk until the mixture is smooth and begins to thicken slightly. Bring to a gentle simmer.
Step 4: Reduce heat to low. Stir in the garlic powder and mustard powder. Continue to cook for 5 minutes, stirring occasionally, until the soup has thickened to your desired consistency.
Step 5: Remove the pot from the heat. Add the grated sharp yellow cheddar cheese, fresh Parmesan cheese, and mozzarella cheese to the soup. Stir continuously until all the cheese has completely melted and the soup is smooth and creamy. Do not return to high heat after adding cheese, as it can cause the cheese to become grainy.
Step 6: Stir in the cooked elbow macaroni pasta until well combined and heated through.
Step 7: Ladle the creamy mac and cheese soup into bowls. Garnish with finely chopped fresh green onions and freshly cracked black pepper before serving hot.