Heat vegetable oil in a large skillet over medium-high heat. Add ground beef and cook, breaking it up with a spoon, until browned. Drain off any excess grease.
Add chopped onion to the skillet and cook until softened, about 5 minutes.
Stir in minced garlic, chili powder, cumin, salt, and pepper. Cook for 1 minute more until fragrant.
Pour in the water and bring to a simmer. Reduce heat and let it simmer for 5-10 minutes, allowing the flavors to meld and the sauce to thicken slightly.
While the beef simmers, warm the taco shells according to package directions.
Spoon the seasoned ground beef mixture into the warm taco shells.
Serve immediately with your favorite taco toppings.
Notes
For a spicier taco, add a pinch of cayenne pepper or a dash of hot sauce to the meat mixture.
You can substitute ground turkey or chicken for ground beef.
Prepare your toppings ahead of time for quick assembly.