Melt butter in a skillet over medium-high heat. Add onions, celery, and carrots. Cook for 5-7 minutes.
Add the chicken thighs, and cook until no longer pink, for 5-7 minutes. Sprinkle flour over the chicken and veggies and cook for a minute
Add rosemary, chicken stock, and pasta. Stir to combine and season to taste with salt and pepper. Cook for 15 minutes or until the pasta is cooked. Add peas and cook for 2 minutes.