How to Make It
In a food processor or on a box grater, coarsely shred the potato and onion. Transfer to a colander and squeeze dry. Let stand for 2 minutes, then squeeze dry again. Transfer the potato mixture to a large bowl. Add the flour, matzo meal, egg, salt and pepper and stir to combine.
In a medium skillet, heat 2 tablespoons of vegetable oil until shimmering. Drop packed teaspoons of the potato mixture into the skillet and flatten them with the back of a spoon. Cook the latkes over moderately high heat until the edges are golden, about 1 1/2 minutes; flip and cook until golden on the bottom, about 1 minute. Drain on paper towels. Repeat with the remaining potato mixture, adding more oil to the skillet as needed.
Arrange the latkes on a platter and top each one with crème fraîche, gravlax and roe. Serve warm.
Paddlefish is a relative of sturgeon. Its roe is available at specialty food stores.