Preheat the oven to 350°. Melt the butter in an ovenproof skillet and sauté the onions until wilted. Add the rice and stir. Add the stock, tomatoes, and lemon. Bring to a boil over high heat. Cover and bake for 40 minutes. Let the rice rest five minutes, or until the liquid is absorbed. Served sprinkled with the toasted pecan pieces.