Heat a large saucepan or skillet over medium-high heat. Add 2 tablespoons olive oil. When oil is hot, add the 1 pound chicken thighs 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 4 tablespoons butter to skillet. Once melted, add 1 cup onion, 8 ounces mushrooms(sliced), and 3/4 cup mini red peppers(diced) and cook, stirring occasionally, until tender, about 10-15 minutes.
Add in 1/2 cup all-purpose flour, stir to prevent burning, and allow to get golden brown and fragrant, about 5 minutes. Add 1/2 teaspoon salt and 1/4 teaspoon pepper and whisk in 1 cup whole milk and 2 1/2 cups 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 10 ounces fresh green peas(steamed). Cook until warmed through. Add additional salt and pepper to taste.
Serve warm over pasta, rice, puff pastry, biscuits, or toast.