For dough, combine flour and salt. Stir in oil, then stir in warm water. Turn dough out onto a lightly floured worksurface and knead until dough is elastic, about 5 minutes.
Cover and set aside while preparing filling.
Boil potatoes uncovered in salted water until tender, then drain well and set aside.
In a large sauté pan, toast fennel, cumin and coriander seeds for 2 minutes (until a fragrance is noticeable). Add oil, then onion and sauté for 4 minutes, until onion is translucent. Add garlic and ginger and sauté one minute more. Stir in spinach, peas and cooked potatoes, mashing lightly to combine and warm, then season to taste. Let filling cool.
To assemble samosas, divide dough into 12 equal portions and shape each portion into a ball. On a lightly floured surface,roll out 1 ball into a 6-inch circle. Cut circle in half. Fold 1 corner of semicircle up and over middle. Fold second corner over to make triangle, and pinch corners of triangle to seal (leave rounded side open). Hold triangle in your hand with open rounded side facing up and let dough fall open to make cone. Fill cone with approximately 2 tablespoons potato mixture, then pinch along rounded side to seal.
Repeat with remaining dough and filling. Cover and chill samosas until ready to cook.
Fill a pot with 2-inches of oil (make sure oil fills pot not more than halfway) and heat to 350 F. With tongs, place samosas in oil, leaving an inch between them, and cook until golden brown, about 4 minutes. Turn over and cook other side until brown, then remove onto a paper-towel lined plate to drain.
Samosas can be served warm or at room temperature with mango chutney.
TIP: Alternately, the samosa can be brushed with an eggwash and baked at 375 °F on a parchment-lined baking tray for 30 minutes.