Step 1

Heat oil in a large skillet over medium heat until hot and shimmering but not smoking; add onion and cook, stirring occasionally, until softened, about 4 minutes. Add spice packet; cook, stirring constantly, until fragrant, about 1 minute. Add ground beef and cook, stirring often, until it is no longer pink, about 5 minutes. (Note: you may need to use a spatula to break up the block of meat into smaller pieces before you can stir frequently.) Add tomatoes or tomato sauce, water, and vinegar; bring to simmer.

Step 2

Reduce heat to medium-low and simmer, uncovered, stirring frequently, until liquid has reduced and thickened (mixture should not be completely dry), about 10 minutes. Taste, adjust seasonings with salt. This can be done up to 5 days (or longer actually) in advance and reheated slowly before serving.

Step 3

If using hard tortilla shells, you can toast them in the oven or toaster oven at 350ºF for about 2 minutes. If using flour tortillas, wrap the tortillas in foil, and place in a 350ºF oven for 15 minutes or until warm. Sometimes I just toast them in the toaster. Remove from oven, and transfer to tea towel to keep warm. Place shredded cheese, lettuce, salsa, sour cream (if using), and avocado (if using) in bowls. Start assembling as you wish.