Thinly slice the potatoes and set aside. Remove sausage from casing and crumble into a medium skillet with onion. Cook for 10 minutes, or until both are browned, breaking up sausage with the back of a spoon.
Stir together ricotta, basil, garlic powder and egg white in a small bowl. Spread ½ cup marinara sauce in the bottom of a 9-inch square baking dish. Place ⅓ of the potatoes in the bottom of the dish, forming a solid layer with no gaps. Drop ½ the ricotta mixture in spoonfuls over the top and spread out just a little. Sprinkle with ⅓ of the mozzarella and ½ the sausage mixture. Add ½ cup more sauce then repeat potato, cheese and meat layers. Top with the last layer of potatoes, remaining sauce and mozzarella.
Cover with plastic wrap and make a small slit to vent. Microwave on HIGH for 30 minutes, or until potatoes are tender.
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