Place the oven rack in the upper third of the oven and preheat it to 400°F.
Grate the potatoes and onion in a food processor with the shredding disk or with a hand grater.
Place in a colander and drain well, squeezing out excess moisture.
In a large bowl, whisk the eggs, salt, melted margarine, and matzah meal.
Stir in the potato-onion mixture until well combined.
To bake, place 2 tablespoons margarine in a 9 x 1 3-inch baking dish.
Melt in the oven or microwave.
Tilt the dish to coat evenly with margarine.
Pour the potato mixture into the dish and spread evenly.
Bake, uncovered, at 400 F for 15 minutes.
Reduce the oven heat to 375°F and continue baking for 45 more minutes, or until the top is crisp.
Cut into squares to serve.