Trim any excess fat from the chicken legs. Heat the oil in a large frying pan until hot. Season the chicken legs all over with salt and pepper, then fry until golden brown, about 2 minutes on each side. Add the thyme, garlic, butter, and water to the pan. Reduce the heat to low, partially cover the pan, and braise until the chicken legs are tender, 30-40 minutes. Turn the legs over halfway through the cooking.

About 15 minutes before the chicken is ready, melt the butter in another pan and tip in the onions. Toss well and cook over medium to low heat, stirring frequently, until the onions are tender, 8-10 minutes. Add the thyme sprigs, peas, and water. Season well. Simmer until the peas are tender and most of the water has evaporated, 5-6 minutes. Add the lettuce and stir until just wilted, about 1 minute longer.

Spread the braised peas and onions on a large serving platter and arrange the glazed chicken legs on top. Serve immediately.