Heat The Oil In A 10-Inch Skillet Over Medium-High Heat. Add The Onion And Chili Powder And Cook Until The Onion Is Tender, Stirring Often. Stir The Soup And Chicken In The Skillet And Cook Until The Mixture Is Hot And Bubbling. Spoon The Chicken Mixture Into The Taco Shells. Top With The Lettuce And Tomato.