HIT THE HEAT
Preheat the oven to 350 degrees. Season the chicken with 1/2 tsp. salt. In a large, ovenproof skillet fitted with a lid, heat the oil over medium heat. Add the chicken and cook, turning occasionally, until browned, about 15 minutes; transfer to a plate. Add the onion to the pan and cook until the edges are golden, about 5 minutes.
Using a wooden spoon, stir in the beer and mustard, scraping up any browned bits from the bottom of the pan.
COVER AND SIMMER
Return the chicken to the skillet, cover and transfer to the oven. Cook, turning once, until a meat thermometer inserted into the thickest parts registers 160 degrees and the sauce begins to thicken, about 25 minutes (as shown above).
FINISH THE DISH
Using a slotted spoon, transfer the chicken to a deep serving dish. Place the skillet over high heat and add the peas. Cook until the peas are tender but still green and the sauce is thickened, about 5 minutes; season with salt and pepper. Spoon the sauce and peas over the chicken. Serve immediately.