For the Tamarind Chutney

5 oz. jaggery or brown sugar, broken into small chunks
4 tbsp. tamarind concentrate
1 tsp. ground cumin
¹⁄₂ tsp. black salt (optional)
¹⁄₂ tsp. garam masala
¹⁄₂ tsp. hot paprika

For the Samosas

3 cups flour
6 tbsp. canola oil, plus more for frying
¹⁄₂ tsp. ajwain seeds (optional)
¹⁄₂ tsp. kosher salt, plus more to taste
¹⁄₂ tsp. nigella seeds (black onion seeds; optional)
2 tsp. coriander seeds, crushed
1 tsp. cumin seeds, crushed
1 tsp. garam masala
¹⁄₂ tsp. ground turmeric
¹⁄₂ tsp. hot paprika
1 ¹⁄₂ lb. russet potatoes, peeled, cut into 1" cubes, and boiled until tender
¹⁄₂ cup frozen peas
3 green Thai chiles, stemmed, seeded, and minced
3 cloves garlic, minced
1 medium onion, minced
1 ¹⁄₂ tsp. fresh lemon juice
Freshly ground black pepper, to taste

Instructions

Make the chutney: Heat jaggery, tamarind, and 2 cups water in a 2-qt. saucepan over medium-high heat. Cook, stirring frequently, until jaggery and tamarind dissolve, 6-8 minutes. Pour liquid through a strainer into a bowl; discard solids. Return liquid to saucepan and stir in cumin, black salt, garam masala, and paprika; cook, stirring occasionally, about 5 minutes. Remove from heat and let cool to room temperature; set chutney aside.
Make the samosas: In a large bowl, stir together flour, 3 tbsp. oil, ajwain, salt, and nigella seeds. Add 1 cup cold water and stir until dough forms. Transfer dough to a work surface and knead until soft and pliable, about 3 minutes. Cover dough with a damp cloth and let rest for 30 minutes.
Heat remaining oil in a 12" skillet over medium-high heat; add coriander, cumin, garam masala, turmeric, and paprika and cook until fragrant, about 30 seconds. Add the potatoes, peas, chiles, garlic, and onions and cook until onions are soft, 5-6 minutes. Remove pan from heat; transfer mixture to a medium bowl. Add lemon juice, season with pepper, and lightly mash, leaving some large chunks of potato intact. Set aside to cool.
Uncover dough, divide into 8 equal pieces, and form each piece into a ball. Place dough on a lightly floured work surface. Using a rolling pin, roll 1 dough ball into a 6" round; cut round in half. Gather straight edges of 1 half-round together, overlapping them by ¹⁄₄" to form a cone; moisten seam with water and press to seal. Spoon 1 heaping tbsp. filling into cone. Moisten inside of top edge of cone with water, press edges together to close top of cone, and pinch along top ¹⁄₄" of seam to completely seal filling in dough cone. Pleat length of seam by folding over about ¹⁄₄" of the dough and pinching it together in about ¹⁄₂" increments. Repeat process with remaining dough. Set filled pastries aside.
Pour oil into a 5-qt. saucepan to reach a depth of 2"; heat over medium-high heat until a deep-fry thermometer reads 360°. Working in batches, fry samosas, turning, until golden brown, 4-5 minutes. Using a slotted spoon, transfer samosas to paper towels to let drain; repeat with remaining samosas. Serve with tamarind chutney.