Dry off the chicken pieces with paper towels. Heat up a large aluminium or iron skillet over a medium flame. Add the oil, brown the chicken then season it with salt and pepper. Degrease the pan. Roast the chicken pieces until done (18-20 minutes). Turn chicken periodically during the roasting process.
Remove the chicken to an ovenproof (pyrex) dish. Degrease the pan, and deglaze it with white wine. Scrape off all of the brown bits (fond) from the bottom of pan. Reduce the wine, until it achieves a reddish color and syrupy consistency. Add the heavy cream and reduce - until the sauce is relatively tight. Stir in the mustard. The mustard will tighten the sauce completely. Do not cook this mustard sauce for more than minute, or the sauce may take on a bitter finish. Pour the mustard/cream sauce over the chicken. Top the chicken with grated Gruyere. Broil until the cheese is melted and bubbly.