PREHEAT oven to 400F. Brush a 9-in. pie plate with oil.
PLACE grated potato in a clean tea towel and squeeze out as much liquid as you can. Transfer to a bowl and stir in egg white and 1/4 tsp salt until combined. Press evenly onto bottom and sides of pie plate. Bake in centre of oven until golden, about 30 min.
MELT butter in a large frying pan over medium-high. Add onions and cook, stirring often, until browned, about 8 min. Reduce heat to medium if they start to burn. Stir in peppers and garlic. Cook for 2 more min. Remove from heat.
WHISK 4 eggs with cream, 1/2 cup parmesan and remaining 1/4 tsp salt in a large bowl. Season with fresh pepper. Stir in onion mixture until combined. Scrape into warm potato crust.
REDUCE oven temp to 350F. Bake in centre of oven until quiche is set, about 35 min. Sprinkle quiche with remaining 1/4 cup parmesan.