Preheat your oven to 200°C (390°F) and line a baking sheet with parchment paper or a silicone mat.
In a skillet over medium heat, toast cumin seeds until fragrant, about 30 seconds. Add chopped onions and cook until translucent, about 5 minutes, filling your kitchen with a warm aroma.
Add mashed potatoes, green peas, garam masala, and a squeeze of lemon juice to the skillet. Mix well, cook for another 2-3 minutes until heated through, then remove from heat and let cool slightly.
Meanwhile, in a mixing bowl, combine flour, salt, and turmeric. Stir in melted ghee until the mixture resembles coarse crumbs.
Gradually add cold water, kneading gently until a firm, smooth dough forms. Cover with a damp towel and let rest for 15 minutes to relax the gluten.
Divide the dough into 6-8 equal portions. Roll each piece into a thin oval about 15cm long on a floured surface.
Cut each oval in half to create semi-circles. Wet the straight edge with a little water to help seal.
Place a spoonful of filling near the straight edge, then fold into a cone shape, sealing the edge with your fingers or a fork.
Place the assembled samosas on the prepared baking sheet. Brush the tops lightly with beaten egg for a shiny, golden finish.
Bake in the preheated oven for 20-25 minutes, rotating halfway through, until the samosas are golden brown and crispy.
Remove from oven and cool slightly on a wire rack. Serve warm with chutney or yogurt for a satisfying snack or meal.
