Place the potatoes in a large pot of salted cold water, enough to cover the potatoes by an inch. Bring the water to a boil, cover the pot, and let the potatoes cook 8-10 minutes until they are fork-tender.
Drain the potatoes and set them aside.
In a small saucepan, heat the oil with the garlic. Saute the garlic until it begins to turn color. Turn off the heat and discard the garlic. Add all of spices-- curry powder, chaat masala, turmeric, paprika -- and the salt and coriander leaves to the oil. Mix well.
Spray a large baking sheet, or two smaller ones, with oil.
Place the potatoes on the sheet, a couple of inches apart. With the heel of your hand, flatten the potatoes, taking care not to break them.
Dab the masala oil on to the potatoes with a brush.
Bake in a preheated 425-degree oven for 45 minutes or until golden on top.
Remove to a plate, sprinkle on the scallions, and serve hot.