Boil potatoes under fork tender. Drain and crumble.
Sauté mustard seeds, cumin seeds and hing in butter and oil until fragrant (around a minute). Add ginger, garlic and chilis and sauté another few minutes until the ginger is fragrant. Add peas and the rest of the spices. Sauté about 5 minutes until peas are cooked. Carefully stir in the potatoes so as not to mash them. Let cool until safe to handle, about 5-10 minutes.
Heat about 2 inches of canola oil in a large, shallow pan to 350. Place a small amount of filling (about 1/4 cup) in the center of the egg roll in sort of a log shape on each wrapper.
Place the wrapper so it like a diamond in front of you then fold each side towards the middle, brushing each edge with egg, and roll towards the top. Repeat until the filling is gone.
Fry until golden, flipping occasionally, drain on paper towel-lined plates.