Preheat oven to 425°F. Grate 1 tablespoon peel and squeeze 1 tablespoon juice from 1 lemon. Combine lemon peel, lemon juice, garlic and pepper in small bowl; reserve 1 tablespoon for potatoes. Press remaining lemon mixture onto all surfaces of beef Tri-Tip Roast. Cut remaining lemon into 12 wedges lengthwise; set aside.
Place roast on rack in shallow roasting pan. Do not add water or cover; set aside.
Combine reserved 1 tablespoon lemon mixture, potatoes, lemon wedges, oil and 1/2 teaspoon salt on rimmed sheet pan; toss to coat evenly. Roast beef in 425°F oven 30 to 40 minutes for medium rare; 40 to 50 minutes for medium doneness. Roast potatoes 25 to 30 minutes or until potatoes are tender, stirring once.
Remove roast when instant-read thermometer registers 135°F for medium rare; 150°F for medium. Transfer to roast to carving board; tent loosely with aluminum foil. Let stand 15 to 20 minutes. (Temperature will continue to rise about 10°F to reach 145°F for medium rare; 160°F for medium.)
Carve roast across the grain into slices; season with salt, as desired. Serve with potatoes and lemon wedges.
Nutrition information per serving: 305 Calories; 105.3 kcal Calories from fat; 11.7 g Total Fat; 3.3 g Saturated Fat; 0.1 g Trans Fat; 0.8 g Polyunsaturated Fat; 6.8 g Monounsaturated Fat; 65 mg Cholesterol; 267 mg Sodium; 26 g Total Carbohydrate; 2.9 g Dietary Fiber; 2.4 g Total Sugars; 25 g Protein; 0 g Added Sugars; 37.2 mg Calcium; 2.7 mg Iron; 1004 mg Potassium; 0 mcg Vitamin D; 0.2 mg Riboflavin; 11.1 mg NE Niacin; 0.8 mg Vitamin B6; 1.3 mcg Vitamin B12; 272 mg Phosphorus; 4.6 mg Zinc; 26.7 mcg Selenium; 110.8 mg Choline;