Warm the oil in a skillet, add the mustard seeds, and cook over a moderate heat until they begin to pop. Stir in the onions and fry for 5 minutes until they are soft.
Add the curry powder and salt, fry for 1 minute. Add the potatoes, carrot, beans, peas, and water. Cook for 15 minutes on a low heat, stirring occasionally, until the vegetables are tender and the liquid almost evaporated.
Remove from the heat and let cool.
Cut the phyllo pastry sheets in half to make long strips. Work with one strip at a time, covering the remaining strips with wax paper or a damp cloth to prevent drying out.
Place a spoonful of the filling at one end of the strip. Fold over the corner diagonally to form a triangle. Continue the folding to the end of the strip. Seal the ends with a little water. Repeat with the remaining strips.
Fill a deep-fryer two-thirds full with sunflower oil and heat to 350°F.
Fry the samosas two at a time for 2–3 minutes until golden. Drain on paper towel, then serve hot or at room temperature.
To make the chutney, peel the mango and slice the flesh from the pit, roughly chop, and set aside.
In a food processor, process the shallot, chiles, and garlic until smooth.
Add the mango and pulse to roughly chop. Season with salt, transfer to a serving bowl, and serve with samosas.