In a saucepan, boil potatoes in salted water 15-20 minutes or until tender. Drain well, return to saucepan and shake over low heat a few moments or until dry. Mash well. Cook peas in boiling salted water 4 minutes. Drain well.
Heat oil in a skillet. Add onion, cumin seeds, ginger, turmeric, Garam Masala and salt. Cook gently 5 minutes. Add mashed potatoes and peas, then stir in lemon juice. Mix well, remove from heat and cool.
Sift flour into a bowl. Cut in butter finely until mixture resembles bread crumbs. Add milk and mix to form a stiff dough. Divide in 6 equal pieces.
Form each piece in a ball and roll each ball on a lightly floured surface to a 6" circle. Cut each circle in half. Divide filling equally among semicircles of pastry.
Dampen edges of pastry, then fold over and seal to form triangles which enclose filling completely.
Half fill a deep-fat fryer or saucepan with oil. Heat oil to 375'F. (190'C. ) or until a 1/2" cube of day-old bread browns in 40 seconds.
Fry samosa in hot oil, a few at a time, 3-4 minutes or until golden brown. Drain on paper towels. Garnish with lime twists and celery leaves if desired.
Serve hot with Mango Chutney.