In a large skillet, heat bacon grease over medium heat. Add beef, onion, garlic, tomato, chiles and potato. Stir in salt, oregano, chili powder and cilantro. Simmer until liquid has reduced, 2-3 minutes.
To assemble, place 1/2 cup off-center on each tortilla. Fold up edge nearest filling; fold in both sides and roll up. If needed, secure with a toothpick.
In an electric skillet or deep-fat fryer, heat oil to 375°. Cook chimichangas until golden brown, turning to cook all sides. Drain on paper towels. Serve with toppings of your choice.