Preheat oven and start biscuits according to package directions if using.
Melt 4 Tbsp butter in a large skillet over medium-high heat.
Add diced onion and minced garlic, sauté 2-3 minutes until softened.
Whisk in 1/2 cup flour and cook 1 minute, stirring constantly.
Slowly pour in 5 cups chicken broth while whisking, about 1/3 cup at a time.
Add 1/3 cup half-and-half and whisk to combine.
Add chicken pieces, 2 cups orzo, frozen peas and carrots, celery, thyme, salt, pepper, and lemon zest.
Stir well and simmer uncovered over medium heat for 20-25 minutes until orzo is tender.
Stir and scrape the pan bottom every few minutes to prevent sticking.
Add remaining broth in splashes if the mixture becomes too thick.
Taste and adjust seasoning as needed.
Serve in bowls with biscuits on the side or crumbled on top.