Heat the oven to 450°F. Line a large baking sheet with foil and lightly coat it with oil or cooking spray.
While the oven heats, prepare the beef. Place a skillet over medium-high heat, add the ground beef and diced onions(if using) Cook while breaking the beef apart until fully browned. Drain off excess grease if needed.
Stir in the taco seasoning and water. Bring to a simmer and cook until the sauce thickens and most of the liquid cooks off about 3 to 4 mins.
Add the black beans to the skillet and cook for about 3 minutes, stirring occasionally, until warmed through. Remove from heat.
Arrange the tater tots in an even layer on the prepared baking sheet. Bake for 20 minutes, flip them, then return to the oven for another 10 minutes until crisp and golden.
Sprinkle the cheese evenly over the tater tots, then spoon the beef and bean mixture on top. Place the pan back in the oven for about 5 minutes, just until the cheese melts.
Finish with tomatoes, sliced peppers, sour cream, guacamole, green onions, and cilantro. Add extra cheese if you like and serve with lime wedges on the side.
Notes
For extra crispy tots: Avoid overcrowding the pan. Use two baking sheets if needed so the tots crisp instead of steaming.
Make-ahead option: The beef and bean mixture can be cooked up to a day ahead and reheated before assembling.
Heat control: Serrano peppers are spicier than jalapeños. Remove seeds or use fewer slices if serving kids.
Serving tip: Serve straight from the baking sheet for a casual spread, or transfer to a platter for parties.