Preheat oven to 400°F. Heat the oil in a pan over medium-high heat and sauté the onion until soft. Add the ginger and cook 2 more minutes. Stir in the peas and potatoes. Add the mustard, coriander, cumin and salt. Cook for 2 minutes. Set aside.
Unroll the pastry dough and cover with a damp kitchen towel to keep it from drying out. Brush 1 pastry sheet with melted butter. Fold the sheet in half and brush with more butter. Fold in half one more time and brush with butter again. Place 1 tbsp. of the cooked filling on one end of the pastry sheet, leaving a 1-inch border around the sides.
Fold the right corner over the filling diagonally and squeeze edges together to form a tightly sealed triangle. Brush the triangle with more butter and place on a baking sheet. Cover while making more samosas. Bake for 30 minutes or until golden brown.