1. Preheat oven to 450°.
2. Heat 2 TBSP of the olive oil over medium-high heat and sauté the chopped onion until it’s translucent, about 8 minutes.
3. Deglaze the pan with the wine and when most of the wine is cooked off add the mushrooms and butter. Reduce the heat to medium and cook until the mushrooms are tender.
4. Add the chicken stock, salt and pepper, and let the sauce cook down until it thickens a little.
5. Meanwhile, in an oven proof sauté pan heat the other 2 TBSP of olive oil.
6. Season the fillets with salt and pepper and add to the hot sauté pan. Cook fillets for about 3 minutes on each side and then transfer to the oven for 3-4 minutes.
7. Spoon some of the onion-mushroom mixture onto plates and top with the pan roasted fillets.