Preheat the oven to 375°F (190°C). Grease a muffin tin or line with paper liners.
In a mixing bowl, combine the flour, softened butter, and salt. Mix until crumbly. Gradually add cold water, mixing until the dough comes together.
Roll out the dough on a floured surface to about 1/8 inch thick. Cut out circles to fit into the muffin tin cups.
Press the dough circles into the muffin tin, forming a crust in each cup.
In a separate bowl, whisk together the eggs and milk. Season with salt and pepper.
Evenly distribute the diced vegetables and cooked meat (if using) among the crusts. Pour the egg mixture over the top, filling each cup about 3/4 full.
Sprinkle the shredded cheese on top of each quiche.
Bake for 20-25 minutes, or until the egg is set and the tops are golden brown. Allow to cool for a few minutes before removing from the tin.