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-lined 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 past; 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, and add roast, browning well on all sides. Place roast over vegetables in cooker. Pour tomato paste mixture into pan, scraping to loosen browned bits. Pour tomato paste mixture over roast and vegetables. Cover and cook on LOW 10 hours or until roast is tender. Place roast and vegetables on a serving platter; keep warm. Reserve cooking liquid in cooker; increase heat to HIGH.
6. Place flour in a bowl. Gradually add water, stirring with a whisk until well blended. Add flour mixture to cooking liquid in cooker. Cook, uncovered, 15 minutes or until slightly thick, stirring frequently. Serve gravy with roast and vegetables.