In a medium bowl, stir together flour, salt, sesame seeds, and celery seeds.

Add butter, and, working quickly with your fingers, cut the butter into the flour mixture until it resembles a coarse cornmeal.

Add water, about a tablespoon at a time, and work in until it forms a dough that sticks together.

Turn out onto a clean work surface and knead about 30 times. Wrap in plastic and let rest for 30 to 40 minutes.

Place potatoes in a medium saucepan and cover with cold salted water. Place over medium-high heat and bring to a low boil. Reduce heat to a simmer and cook until potatoes are tender, about 10 minutes. Drain completely.

Meanwhile, in a small bowl, mix together coriander, cumin, turmeric, cayenne, and ginger with about 1 tablespoon of water to form a paste. Set aside.

Heat oil in a large skillet over medium heat. When hot, add onions and cook for 3 minutes. Add spice mixture, and stir and cook for about a minute. Add about 1/4 cup of water and cook, scraping the bottom of the pan to loosen the spices that have stuck.

Add peas and potatoes, stir to coat, and heat through. Taste and add salt as needed.

Roll dough into a rough rectangle about 20x12 inches and 1/8 inch thick. Use a large cutter or the lid of a small saucepan to cut 4-inch diameter circles.

Place 2 Tbs of filling in the center of each circle, brush water around the edge, fold over, and seal closed, using a fork to crimp. Place on a baking sheet about 2 inches apart.

If you wish, you can place the samosas closer together and freeze for an hour, then transfer to a plastic bag for longer-term storage in the freezer. Bake from frozen as below, but increasing the time by 10 to 15 minutes.

The dough can be re-rolled once or twice.

Preheat oven to 350°F.

Bake until crusts are brown and everything is heated through 30 to 40 minutes.