Heat butter and saute onions until brown. Add wine, bring to a boil and reduce all the liquid. Add stock, peppercorns and bay leaves. Reduce by half. Strain and keep warm. Brown mushrooms in butter, add shallots and brown. Add thyme, garlic, vinegar, salt and pepper. Remove from heat and allow to marinate for 20 minutes. Preheat oven to 400 degrees. Season filets with salt and pepper. Pour oil in ovenproof skillet. Sear both sides of the meat. Then place in oven for 10 minutes. Bring mushrooms and sauce back to a serving temperature. Place filet on plate, top with sauce and mushrooms.
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