In a large saucepot over medium-high heat, melt butter. Add leeks, onions, celery, and a large pinch of salt. Cook until onions are translucent and celery is soft. Add mushrooms and rice and stir well. Add vegetable broth and pecans, then bring to a boil and cover. Reduce heat to low and cook until rice is tender, about 15 to 20 minutes. Remove lid and turn off heat. Let stand for 15 minutes, then add parsley and chives, fluff rice with a fork, and serve.