Dough: In bowl, whisk flour with salt; grate in frozen butter. In measuring cup, combine vinegar with enough ice water to make ? cup; slowly drizzle into flour mixture. Bring together with hands just until dough holds together but is still crumbly. Form dough into disk; wrap in plastic wrap. Refrigerate for 30 minutes.
Filling: In bowl, whisk together eggs, mixed vegetables, milk, garlic, chives, parsley, mustard, paprika, pepper and salt. Stir in feta cheese.
Preheat oven to 350°F (180°C). On lightly floured surface, roll out pastry to ?-inch (3 mm) thick circle. Using 4-inch (10 cm) round cutter, cut out 12 rounds, re-rolling scraps once. Place rounds into 12-cup muffin tin and press down gently. Divide filling among pastry cups. Bake for 30 to 35 minutes or until pastry is golden and filling is set.
• To make ahead, fill pastry with mixture, then freeze.
• Use store-bought pie crusts for a time-saving hack.