Heat the oil in a small nonstick pan. Add the mustard seeds and fry for about ten seconds, or until they begin to splutter.
Add the onion and ginger, and cook for 2 to 3 minutes over a high heat. Add the peas, and stir well. Add the coriander, cumin, chile powder, garam masala, mango powder, salt to taste, and a splash of water. Cook for 1 to 2 minutes, and then add the potatoes, and cilantro, and cook for another 2 to 3 minutes. Taste the mixture and adjust the seasoning as necessary.
Preheat the oven to 400 degrees F.
Unroll the phyllo pastry dough, cover with plastic wrap and a damp tea or kitchen towel. Peel off one sheet and keep the rest covered so that it doesn't dry out. Lay the pastry sheet flat on a clean surface, and brush with melted butter. Fold in one third of the pastry lengthwise towards the middle. Brush again with the butter and fold in the other side to make a long triple-layered strip.
Place one rounded teaspoon of the filling mixture at one end of the strip, leaving a 1-inch border. Take the right corner and fold diagonally to the left, enclosing the filling and forming a triangle. Fold again along the upper crease of the triangle. Keep folding in this way until you reach the end of the strip. Brush the outer surface with more butter. Place the triangle onto a baking sheet and cover while you make the rest of the samosas. Sprinkle over a few sesame seeds, if using.
Bake in the center of the oven for 30 to 35 minutes, or until golden and crisp, turning halfway through the cooking time.
To serve, place the samosas onto a large serving plate with a bowlful of chutney.
* Can be found in specialty Asian markets.