To make the dough, combine flour and salt in large mixing bowl, then stir in oil. Add water, little by little, to form a workable dough. Form the dough into a ball and wrap in a damp kitchen towel to rest at room temperature for 30 minutes.
To make the filling, place a deep, heavy-bottomed saucepan over medium-high heat and add the oil. When oil is hot, add the onion, ginger, garlic, and 1/2 tsp salt and stir. Cook for 2 minutes or until onion softens. Add minced chicken, remaining 1/2 tsp salt, cumin, coriander and chili powder. Cook 10 minutes, stirring occasionally, then sprinkle with garam masala and add frozen peas. Stir and cook 5 more minutes. Taste for salt and add more if needed. Add chopped chillies if using (filling will be mild without).
To shape the samosas, divide the dough into 6 equal portions, then roll each into a smooth ball on a well-floured surface. Roll each ball into a thin, oblong shape, then use a knife to slice in half lengthwise to make a half-moon. Take one half in your hand, brush some water on the edges, and fold into an open cone shape, pressing edges to seal. Hold the cone and fill with about 2 tbsp chicken filling. Pinch to seal Allow it to sit on a cutting board to form a flat bottom. Finish shaping remaining samosas.
To fry the samosas, pour the oil into a deep saucepan or dutch oven to a depth of 3" over medium heat. Fry the samosas in batches once the oil reaches about 200ºF. Fry for 15 minutes until golden, then increase heat to high and cook for 5 minutes at a hotter temperature until deep golden brown. Remove with a slotted spoon and place on paper towels to drain.