30 minutes before cooking, remove the chicken from the fridge and finely dice the onion.
Pre-heat your saute pan to medium high. Add 3 tablespoons of olive oil to the hot pan. Make sure the pan surface is completely coated by tilting the pan back and forth. When the pan starts to smoke just a little bit, it is ready. Carefully add the chicken to the pan and lower the heat to medium. Because this is a cream sauce, you don't want to sear the meat too dark or caramelize the onions too brown. This prevents the drippings from giving the sauce too dark a color. Saute the chicken 2 minutes per side until golden brown and transfer to a plate. Discard any excess oil in the pan leaving just a little to cook the onions.
Add the onions and 1 tablespoon of fresh thyme leaves and cook gently until golden brown, about 5 minutes.
Add ½ cup of white wine and using a wooden spoon or wooden spatula, scrape any drippings stuck to the pan. (Deglazing) Reduce this liquid to a syrup and add another ½ cup of wine. Reduce the additional wine to an essence and add ½ the heavy cream. Reduce the heat to medium low and simmer the cream and onions until reduced by half. Add the remaining ½ cup of cream and the whole grain mustard. Using your wooden spoon, stir until the sauce is completely incorporated.
Add your remaining fresh thyme leaves.
Adjust the seasonings, add the chicken and any accumulated juices back to the pan and cover.
Simmer, turning once until completely cooked.