Drain cans of roast beef of juice and, with fingers, separate the chunks into shreds.
Heat a generous amount (3-4 Tablespoons) of bacon grease in frying pan over medium heat and add onions and jalapeno and fry just until both are tender. Add shredded roast beef, garlic powder and cumin. Cook, covered, over low heat, for 15-20 minutes.
In saucepan or frying pan, heat about 1/2 inch of corn oil over medium-high to high heat. With tongs, dip each tortilla in hot oil and fry until it begins to blister and become limp. When done, remove tortillas to a plate covered with a paper towel.
Place about 2 Tablespoons of roast beef filling on each tortilla and roll the tortilla around the filling to form a cylinder. Secure the the open flap of the tortilla with a toothpick.
Place the flautas on a rack over a cookie sheet and bake in 400 degree oven for 15 minutes or so, or until they are slightly browned and crispy. Remove toothpicks and serve flautas with side of guacamole, sour cream, and/or tortilla chips and salsa.
Note: The filling can be made a day ahead of time and kept in a casserole dish in the refrigerator. Just before frying the tortillas, microwave the filling on high for about 2-3