Preheat oven to 425°F (218°C). In a large skillet over medium-high heat, cook ground beef, breaking it apart, until no pink remains, 6 to 8 minutes. Drain excess fat, then stir in taco seasoning and water. Simmer until thickened, about 3 minutes.

Brush both sides of corn tortillas lightly with oil. Line a sheet pan with parchment. Arrange tortillas on the pan, spoon beef onto one half of each, top with cheese, then fold the tortilla over and press down gently.

Bake 10 to 12 minutes, until tortilla edges are golden and cheese is bubbly. For flour tortillas, check at 8 minutes.

Let tacos rest on the pan 2 minutes, then transfer to a plate. Open each taco slightly and stuff with lettuce, tomatoes, sour cream, and salsa. Serve immediately.