Preheat the oven to 375°F (190°C).
On a lightly floured surface, roll out the pie crust dough into a circle slightly larger than a 9-inch pie plate. Fit the dough into the pie plate, trimming any excess, then flute or crimp the edges as desired. Place the crust in the refrigerator to chill while you prepare the filling.
In a medium nonstick skillet over medium heat, add the ground beef and diced onion. Cook, breaking up the beef as it browns, until the meat is cooked through and the onion is soft, about 8-10 minutes. Drain any excess grease.
Add the taco seasoning and water to the skillet with the beef mixture. Stir until fully combined, and cook until most of the water has evaporated.
Spread the beef mixture evenly over the prepared pie crust. Top with the Monterey Jack and cheddar cheeses.
Bake for 22-27 minutes, or until the crust is golden and the cheese is fully melted.
Remove the pie from the oven, and sprinkle the top with crushed Doritos. Serve with salsa, sour cream, diced tomatoes, shredded lettuce, and chopped cilantro.