Cut slits in top of roast; insert garlic slices. Combine the oil, paprika, pepper and thyme; rub over roast. Place in a large resealable plastic bag; seal and refrigerate the roast overnight.
Transfer roast to a shallow roasting pan. bake, uncovered, at 350° for 1-3/4 hours. Meanwhile, place potatoes and salt in a saucepan and cover with water. Bring to boil. Reduce heat; simmer, uncovered, for 15 minutes or until almost tender. Drain; cool slightly. Cut potatoes into quarters; arrange around roast.
Bake 45 minutes longer or until a meat thermometer reads 160° and potatoes are tender, basting potatoes with drippings occasionally. Remove potatoes; keep warm. Cover roast and let stand for 15 minutes before carving.
For gravy, pour drippings and loosened browned bits into a measuring cup. Skim fat, reserving 2 tablespoons drippings. Add water and broth to reserved drippings; set aside. In a large saucepan, saute onions and mushrooms in butter and oil until tender.
Stir in flour until blended. Gradually stir in broth mixture. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in parsley and pepper. Serve with roast and potatoes.