Season 4 whole skinless, boneless chicken thighs with kosher salt and freshly ground black pepper an hour before cooking.
Heat 2 tbsp oil in a medium deep sided skillet over medium high heat. Add chicken and cook until golden brown on both sides. About 4-5 minutes per side. Remove chicken to a platter and keep warm.
Add 1 small finely diced onion and 4-5 large thinly sliced Cremini mushrooms to the skillet and sauté until the mushroom, let out their liquid and begin to brown, about 10 minutes.
Add 2 cloves grated garlic and 1 tsp thyme and stir to combine. Sauté until the garlic becomes fragrant, for about 2 minutes
Deglaze the pan with 1/4 cup chicken stock, scraping up brown bits.
Add the remaining 1/2 cup chicken stock, 1 tbsp grainy mustard, 1 tbsp Dijon mustard and stir to combine. Add 1/2 cup grated Asiago cheese and stir until melted and incorporated.
Add 1/2 cup heavy cream stir to combine and return chicken to the skillet and allow sauce to thicken.
