Combine onion, olive oil, and garlic in a large pot over medium heat. Cook and stir for 2 minutes. Add chicken breasts; cook until lightly browned, about 2 minutes per side. Pour in water; bring to a boil.
Pour yellow rice into the boiling water. Cover and simmer over medium heat until chicken and rice are tender, about 20 minutes. Sprinkle frozen peas on top; cook until thawed, about 2 minutes. Mix up rice.