Cook and stir ground beef, onion, and jalapeno peppers together in a skillet over medium-high heat until meat is browned and crumbly, 7 to 10 minutes. Stir taco seasoning into meat mixture; bring to a simmer and cook until flavors combine, about 5 minutes.
Stir meat mixture, shredded lettuce, tomato, Cheddar cheese, salsa, and sour cream together in a large bowl. Divide taco mixture among 4 bowls and top each with avocado slices.
