Combine salt, thyme and pepper; rub over roast. Place fat side up on a rack in a shallow roasting pan. Bake, uncovered, at 350° 3 hours or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Transfer to a warm serving platter. Let stand 20 minutes.
Meanwhile, for gravy, pour pan drippings and loosened browned bits into a 4-cup measuring cup. Skim fat, reserving 1/4 cup; set drippings aside. In a skillet, saute the mushrooms and onion in reserved fat until tender. Add tomato paste; cook and stir until combined. Remove from the heat.
Add enough beef broth to reserved drippings to measure 3-1/2 cups. In a large saucepan, combine flour, wine, lemon juice and broth mixture until smooth. Stir in mushroom mixture. Bring to a boil; cook and stir 2 minutes or until thickened. Season with salt and pepper. Slice roast and serve with gravy.