In a large heavy-bottomed pot with a lid or Dutch oven, melt 1 tablespoon of the butter over medium heat. Add the onion and celery. Cook for 4 to 5 minutes, until softened.

Add the thawed peas and carrots and cook for 2 to 3 minutes until warmed through. Stir in the garlic and cook for 1 more minute.

Add the remaining 2 tablespoons of butter and the uncooked orzo. Stir to coat and lightly toast the orzo until it just starts to turn golden, about 2–3 minutes, stirring frequently.

Pour in the chicken broth and bring to a simmer. Once simmering, cover the pot with a lid and reduce the heat to low. Cook for about 10 to 12 minutes, stirring occasionally to prevent sticking, until the orzo is mostly tender and most of the liquid is absorbed.

Stir in the shredded rotisserie chicken and heavy cream. Cover again and cook for another 3 to 4 minutes, until everything is heated through and the orzo is fully cooked.

Remove from heat. Stir in the parmesan cheese, salt, black pepper, thyme, and red pepper flakes (if using). If the mixture seems too loose, let it sit uncovered for 2–3 minutes, stirring occasionally, until it thickens. If too thick, add a splash of warm broth or cream to loosen it.

Serve warm, garnished with chopped parsley and extra parmesan.