Heat oil in a pan over medium heat. Add the sliced onion and cook until it turns golden and starts to leave browned bits on the bottom of the pan, 9-12 minutes.
Add the undrained canned beef and stir to heat it through. Mix in barbecue sauce, shred the beef, and stir. Heat on high until the liquid has reduced into a thick sauce, about 5-7 minutes. Serve on hamburger buns with cheese, shredded lettuce, and any other condiments, if desired.
Notes
For added veggies, try shredding a zucchini or carrot and adding it in with the undrained beef. You may need to increase the length of simmering time.