Add the ground beef and onion to a large skillet over medium heat. Cook until browned, stirring occasionally breaking up any large pieces.
Drain off extra grease and add in the taco seasoning, salsa and water. Bring to a light simmer adding more water as desired.
Serve in taco shells as-is or place in a 9x13" dish to bake. Then add cheese and bake at 400°F for 10 minutes.
Add desired toppings.
Notes
Optional: You can also add 1-2 Tablespoons of refried beans to the base of the tacos. If I'm adding them, I like to place them in a zip top bag and then snip the top off and place it on the bottom of the shell. This recipe should make enough for about 2 Tablespoons of meat in each taco. You can mix in some cooked rice and black beans to stretch the meat even further.