Prepare the Ingredients: Cut the chicken breasts into 1-inch pieces, halve the cherry tomatoes, thinly slice half of a red onion, and mince three cloves of garlic.
Sauté Aromatics and Chicken: Heat olive oil in a large skillet over medium-high heat. Add the sliced red onion and cook for 3-4 minutes until softened. Stir in minced garlic and optional red pepper flakes, cooking for another minute.
Brown the Chicken: Add chicken pieces to the skillet, season with salt and black pepper, and cook for 5-7 minutes until browned.
Add Remaining Ingredients: Pour in chicken broth, stir in orzo and cherry tomatoes, bring to a boil, then reduce heat to simmer for 15-20 minutes.
Stir in Pesto and Finish: Remove from heat and stir in pesto and lemon juice until well combined.
Add Parmesan and Serve: Stir in grated Parmesan cheese, adjust seasoning, and garnish with fresh basil if desired before serving.