Heat 2 tablespoons oil in a large saucepan over medium-high heat. Add beef; cook until evenly brown. Stir in potatoes, carrots, onion, bell pepper, water, garlic, salt, and black pepper; bring to a boil. Reduce heat; simmer, about 2 hours. Dissolve bouillon into beef mixture; stir in tomatoes.

Preheat the oven to 325 degrees F (165 degrees C).

Cut top off pumpkin, remove seeds and pulp, and transfer to a heavy baking pan. Fill pumpkin with beef mixture; brush outside of pumpkin with remaining 1 tablespoon oil.

Bake in the preheated oven until tender, 2 hours. Serve stew from pumpkin, scraping out some pumpkin flesh with each serving.