For the pastry: Combine the flour and salt in a large bowl. Rub the grapeseed oil into the flour mixture until well incorporated. Add the ajwain and nigella seeds. Gradually add the water, mixing after each pour, until a smooth dough forms. Knead for 5 minutes. Drizzle the bowl with a little oil; turn the dough to coat. Wrap the dough in plastic and set aside on the counter.
For the filling: In a medium skillet over medium heat, saute the red onions, chiles, ginger and turmeric in grapeseed oil until softened and slightly browned, 6 to 8 minutes. Add the cranberries and peas; cook just until the peas are thawed, a couple minutes. Season with salt.
Transfer the mixture to a bowl with the potatoes and turkey and sprinkle with some lime juice and cilantro. Stir together, then taste for seasoning.
To make the samosas: Heat 3 inches of oil in a Dutch oven over medium until the oil reaches 375 degrees F on a deep-frying thermometer.
Divide the dough into 9 balls. Roll each ball into a 5- to 6-inch round. Slice each round into two semicircles. Set up a small bowl with water. Wet the straight edge a semicircle with a little water, then set the semicircle on a work surface with the straight edge facing away from you. Place about 2 teaspoons of filling in the middle of the semicircle. Fold one corner over the filling, forming a triangle; fold in the other corner to make a cone. Seal the ends with water, pinching and folding the edges to seal tightly. Repeat with the remaining semicircles.
Fry the samosas in batches until golden brown, 5 to 7 minutes. Drain on a wire rack set over a baking sheet on a paper-towel lined plate. Sprinkle with flaky sea salt.
For the green chutney: Add all the ingredients except for the creme fraiche to a blender. Blend until smooth. Add the creme fraiche and blend until incorporated.
Serve the samosas hot or at room temperature with the chutney.