Add your ground beef, chopped onion, and minced garlic to a large frying pan over medium heat. Let it cook for 5-10 minutes, breaking up the beef as it browns. I always keep an eye on the onion here so it softens without burning. You're looking for the beef to lose its raw color completely before moving on.
Stir in the cumin, chili powder, paprika, salt, and pepper, then add the black beans and beef broth. Let this simmer for about 10 minutes until most of the liquid has cooked away. I finish it with the sugar and balsamic vinegar for a touch of balance. Always taste it here — this is your last chance to adjust the seasoning before it goes into the tortillas.
Lay a tortilla flat and add a small handful of grated cheese followed by a spoonful of the beef filling. Roll it up tightly so it holds its shape once cooked. I like to place them seam-side down on a tray as I go so they don't unravel. Repeat until all your filling is used up.
Arrange the rolled taquitos in your air fryer basket or on a lined oven tray, then spray them lightly with cooking oil. Cook at 180°C (350°F) for 8-10 minutes, turning halfway, until they're golden and crunchy on the outside. I always check the ends first since that's where they crisp fastest. If you're using an oven, keep an eye on them since timing can vary slightly.
While the taquitos cook, add the avocado, lime juice, olive oil, garlic, cilantro, chili flakes, salt, and pepper to a blender. Blitz until smooth and creamy, scraping down the sides if needed. I like mine slightly thick so it clings to the taquitos when dipped. Taste and adjust the lime or salt to your liking.
Once your taquitos come out golden and crisp, transfer them straight to a serving plate. Pour the avocado sauce into a small bowl alongside them for dipping. I always serve these immediately since the crunch fades the longer they sit. Dig in while everything's still warm and melty inside.