Heat a large saucepan or skillet over medium-high heat. Add olive oil. When oil is hot, add chicken and cook until golden brown on both sides, about 10-20 minutes.
Remove chicken from pan and set on plate. Loosely tent with foil.
Add butter to skillet. Once melted, add onion, mushrooms and bell pepper and cook, stirring occasionally, until tender, about 10-15 minutes.
Add in flour, stir to prevent burning, and allow to get golden brown and fragrant, about 5 minutes. Add salt and pepper and whisk in milk and chicken stock. Cook, stirring constantly, until a smooth gravy forms.
Bring to a low simmer, stir and cook for 5 minutes. If mixture gets too thick, whisk in a little bit of chicken stock at a time. Meanwhile, chop chicken into bite sized pieces.
Stir in cooked chicken, as well as any juices from the plate, along with the peas. Cook until warmed through. Add additional salt and pepper to taste.
Serve warm over pasta, rice, puff pastry, biscuits, or toast.