Combine chopped tomatoes, chopped cilantro, lime juice, and corn in a medium bowl.

Mix well until combined.

Cover and refrigerate for one hour to blend the flavors.

Heat oil in a large sauté pan.

Cook chopped onions on medium heat until just starting to brown.

Reduce heat to medium-low and caramelize the onions for 20 minutes, stirring occasionally.

Turn the heat back up to medium and add diced garlic.

Cook until fragrant and starting to brown.

Add the ground beef and brown over medium heat.

Drain the beef mixture and rinse to remove fat.

Return the beef and onions to the pan.

Add the black beans and taco seasoning.

Heat and stir for 4-5 minutes.

Place 1/4 cup shredded lettuce in the bottom of each bowl.

Add the meat mixture, then a sprinkle of shredded cheddar cheese.

Add the salsa mixture, then the chopped avocado, and one tablespoon of sour cream.

Serve immediately.