Heat 1 Tbsp oil in a skillet over medium-high heat. Add ground beef and onion, breaking up the beef. Cook until beef is browned and onion is softened, about 5-7 minutes. Drain excess grease.
Stir in the taco seasoning and 1/2 cup water. Bring to a simmer and cook for 2-3 minutes until the sauce has thickened, stirring occasionally.
Remove from heat. Stir in the shredded cheddar cheese and 1/2 cup sour cream until the cheese is melted and the mixture is well combined. Set aside.
Warm the tortillas slightly to make them pliable.
Spoon about 2 tablespoons of the beef mixture onto the center of each tortilla. Roll up tightly, tucking in the ends if possible. Place seam-side down.
Lightly spray the taquitos with oil. Place them in the air fryer basket in a single layer. Air fry at 380°F (190°C) for 6-10 minutes, flipping halfway through, until golden brown and crispy.
Serve immediately with your favorite toppings like guacamole, salsa, sour cream, shredded lettuce, and lime wedges.