Wash hands with warm, soapy water for 20 seconds.
In a large skillet, heat oil on medium high. Add diced onion and cook for 3-5 minutes.
Add beans (drained, if using canned), corn (drained, if using canned), garlic powder, and taco seasoning. Add a little water or salsa, if mixture is too dry.
Cook for 5-10 minutes, until heated through and flavors are combined.
Heat tortillas, if desired. Top each tortilla with ⅓-½ cup of bean mixture. Top with tomatoes, cabbage or lettuce, cheese, and any desired toppings.