Heat some olive oil in a large pan on medium-high heat. Season the 4 chicken breast fillets with salt and pepper. Fry for 3-4 minutes on each side until golden brown.
While the chicken cooks, clean the red, yellow, and green peppers and cut them into strips. Peel the onion and cut it into rings.
Once the chicken is golden brown, add the peppers and onion to the pan. Fry for 1-2 minutes.
Pour in the vegetable stock and sherry. Cover the pan and simmer on medium heat for 15 minutes.
Check that the chicken has reached an internal temperature of 74°C (165°F). Taste the sauce and adjust seasoning with salt and pepper as needed.
Serve with couscous or rice prepared according to package instructions.