Place potatoes and 1 teaspoon salt into a large pot and cover with water; bring to a boil. Reduce heat to medium, stir in garlic powder, and simmer until tender, about 15 minutes. Drain; mash potatoes until no lumps remain.
Heat oil in a skillet over medium-high heat. Cook and stir onion in hot oil until golden and tender, 5 to 10 minutes.
Stir onions, 1/4 teaspoon salt, and pepper into mashed potatoes. Cool.
Preheat oven to 350 degrees F (175 degrees C). Grease a 9x9-inch baking dish.
Stir 4 eggs, matzo meal, and a pinch of salt into potato mixture. Spread potato mixture into prepared baking dish and brush top with beaten egg.
Bake in the preheated oven until a knife inserted into the center of the kugel comes out clean, about 30 minutes.