1. Preheat oven to 400 degrees F. Place squash cut-side up on a baking sheet and bake until tender (20-25 minutes).
2. While squash bakes, heat oil in a large skillet over medium-high heat until hot. Add onion, carrot, and garlic, and cook until vegetables are tender (15 minutes). Add beans, broth, salt, pepper, and 2 teaspoons chopped sage. Heat to boiling, cover skillet, reduce heat to low, and keep warm.
3. Place squash halves, cut side up, on platter. Fill each half with one-fourth of bean mixture. Sprinkle with diced tomato and remaining 1 teaspoon chopped sage.