Combine garlic, thyme, oil, wine, bay leaf, Kitchen Bouquet and soy sauce. Pour over tenderloin and marinade overnight. Remove tenderloin from refrigerator about 30 minutes prior to roasting to allow meat to relax.
Preheat oven to 425. Place tenderloin in open roasting pan and bake 25-30 minutes or until internal temperature reaches 125 (medium rare). Baste with marinade 2-3 times during roasting.
Let roast rest loosely covered and while it rests, pour marinate into small saucepan and boil.
In separate pan, melt butter and saute onion and mushrooms until onions are translucent and mushrooms give off their juices. Add flour to mixture and stir. Pour boiling marinade into pan with mushrooms and onions and stir to thicken. As mixture comes to a boil, reduce heat.
Slice meat and add mushroom ragout to the side or on top of beef.