Heat oil in a large, nonstick skillet set over medium-high heat. Crumble in ground beef; season with steak spice. Cook, breaking up into small pieces, for 5 to 7 minutes or until browned. Add onion, green pepper and mushrooms. Cook, stirring occasionally, for 5 to 7 minutes or until vegetables are tender-crisp.
Stir in tomatoes, soup and mustard. Reduce heat to medium-low. Simmer for 5 minutes or until slightly thickened. Stir in cooked macaroni and 1 cup (250 mL) cheese until well combined.
Sprinkle the top with remaining cheese and green onion. Cover and remove from heat. Let stand for 5 minutes or until cheese is melted.