1. Preheat oven to 425°. Toss together potatoes, oil, and 1/2 tsp. salt in a bowl. Spread potatoes in a single layer in a jelly-roll pan, and bake 35 to 40 minutes or until golden and tender.
2. Meanwhile, stir together sirloin, egg, pepper, and remaining 1 tsp. salt in a large bowl. Shape into 6 (3 1/2-inch) patties.
3. Cook patties in a large skillet over medium-high heat 4 to 5 minutes on each side or until browned. Remove patties, reserving drippings. Sauté onion in hot drippings 2 minutes. Add tomatoes, bouillon, and 1/2 cup water. Cook, stirring occasionally, 3 to 4 minutes or until bubbly.
4. Return patties to skillet; spoon tomato mixture over patties. Cover and cook 15 minutes; uncover and cook 5 minutes. Place 1 cheese slice on each patty. Cover and cook 3 minutes. Sprinkle with basil, if desired. Serve with potatoes.