1. Combine roast, soy sauce, and garlic in a large zip-top plastic bag; seal bag, and marinate in refrigerator at least 8 hours, turning bag occasionally.
2. Bring broth to a boil in a small saucepan; add mushrooms. Remove from heat; cover and let stand 20 minutes. Drain mushrooms through a cheesecloth-lines colander over a bowl, reserving broth mixture.
3. Remove roast from bag, reserving marinade. Sprinkle roast with pepper, gently pressing pepper into roast. Combine reserved marinade, mushroom broth mixture, and tomato paste; stir well, and set aside.
4. Place mushrooms, onion, carrot and potato in a 6-quart electric slow cooker; toss gently.
5. Heat oil in a large skillet over medium-high heat. Add roast, browing well on all sides. Place roast over vegetables in slow cooker. Pour tomato paste mixture into pan, scraping to loosen browned bits. Pour tomato paste mixture over roast and vegetables. Cover with lid; cook on high-heat setting 1 hour. Reduce to low-heat setting, and cook 8 hours or until roast is tender. Place roast and vegetables on a serving platter; keep warm. Reserve liquid in slow cooker; increase to high-heat setting.
6. Place flour in a small bowl. Gradually add water, stirring with a whisk until well blended. Add flour mixture to liquid in slow cooker. Cook, uncovered, 15 minutes or until slightly thick, stirring frequently. Serve gravy with roast and vegetables.