In a saucepan over medium heat, melt butter. Add onion and cook until edges begin to lightly brown, about 4 to 5 minutes. Add garlic and cook for another 30 seconds.
Add orzo, broth, salt, and pepper. Bring to a simmer, cover, and reduce heat to low. Cook until al dente according to pasta's package directions. Most of the liquid should be absorbed.
Remove lid and add heavy cream and parmesan cheese. Stir until cheese has melted and sauce looks creamy.
Top with fresh parsley and serve.
If sauce looks thin, remove from heat and allow to set for 5 minutes before serving.
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Notes
Storage: Leftovers will keep for 3-4 days in the refrigerator.
Freshly grated cheese: Use freshly grated cheese for a smooth sauce. Pre-grated cheese are coated in anti-clumping powders that add a grainy texture when melted.
Measure exactly: Just enough liquid is added to the recipe to cook the pasta. Adding too much liquid or not enough pasta will result in a runny sauce. Allow finished pasta to sit, uncovered, for up to 5 minutes to allow thin sauce to thicken.