In a medium bowl combine the fennel, coriander, sesame seeds, pepper and salt.
Slice the yellowtail into two pieces, each roughly 1 1/2-by-1 1/2-by-6-inches. Generously rub the seasoning on all sides.
Heat the canola oil in a large nonstick saute pan set over high heat. Once the oil starts to smoke a bit, carefully add the yellowtail. Sear each piece on all four sides until browned, 30 seconds to 1 minute per side. The center of the fish should still be rare. Place the cooked yellowtail on a plate to cool, then refrigerate until cold, about 1 hour. The fish will be easier to slice if it's cold.
In a medium bowl combine the creme fraiche, horseradish and lemon zest.
Using a sharp knife, gently slice the yellowtail in 1/2-inch-thick pieces. Spread a spoonful of the horseradish crema on top of the rice crackers. Place the sliced yellowtail on top of the crema. Break off small pieces of the nori and place on top of the yellowtail to garnish.