1. In a large nonstick skillet, heat broth over medium-high heat. Add bell pepper, carrot, celery, and chopped green onion. Cook, stirring frequently, until vegetables are tender, about 10 minutes.
2. Add beef, salt, and pepper to pan. Cook, stirring frequently until beef is browned, about 6 to 8 minutes. Drain off fat. Remove from heat.
3. Preheat oven to 350°F. Place half of beef mixture in bottom of a 9 or 10-inch baking dish. Place sliced tomato over beef, reserving 2 slices. Cover tomato layer with remaining beef mixture.
4. Bake casserole until heated through, about 25 minutes. Garnish with reserved tomato slices and cashews. Serve immediately.