I start by heating a tablespoon of oil in a large pan over medium heat. Once it's shimmering, I toss in the chopped onions and let them cook, stirring occasionally, until they turn soft and slightly translucent. This usually takes about three to four minutes and builds a nice flavor base for the beef.
Next, I add the ground beef right into the pan with the softened onions. Using a large spoon, I break the meat up into small crumbles as it cooks. This step takes a little patience, but smaller pieces of beef roll into the tortillas much more evenly.
Once the beef is mostly cooked, I stir in the taco seasoning, salt, and tomato paste until everything is evenly coated. If the mixture looks dry, I splash in a little water or beef broth to loosen it up. I keep cooking until the beef is completely done, usually another six to eight minutes.
This step is easy to skip, but I never do. I pull the pan off the heat and let the filling cool for several minutes before assembling the taquitos. Warm filling makes the tortillas soggy and much harder to roll tightly.
On a clean surface, I lay out a tortilla and sprinkle three to four tablespoons of shredded mozzarella down the center. Then I spoon about two tablespoons of the cooled beef filling right on top of the cheese, keeping it centered so nothing spills out the sides.
Starting from the edge closest to me, I roll the tortilla into a tight log, tucking as I go so the filling stays locked inside. I place each rolled taquito seam-side down on a baking sheet and repeat until I've used up all the tortillas, cheese, and beef.
I heat about half a cup of oil in a frying pan over medium heat, then fry the taquitos in small batches, seam-side down first. Using tongs, I rotate them every minute or so until every side turns golden brown and crispy, which takes about one to two minutes total per batch.
As each batch finishes frying, I transfer the taquitos to a wire rack set over a paper-towel-lined baking sheet. This lets any extra oil drip away instead of soaking back into the crispy shell, so they stay perfectly crunchy until serving.