Add rice and stir. Reduce heat, cover and simmer until tender, about 40 minutes.
Heat butter in a large saucepan over medium heat.
Saute onion until golden brown; add mushrooms and celery. Cook 2 minutes, stirring constantly.
Reduce heat to low; stir in flour and cook, stirring constantly, until mixture is bubbly. Gradually add broth; increase heat to medium-high and bring to a boil. Boil, stirring, for 1 minute.
Reduce heat to low and add cooked rice, half and half, sherry, salt, white pepper, curry powder, dry mustard, paprika and chervil. Simmer until heated through.