Place the bread, onion, and garlic in a food processor and pulse on the off about 5 times. Add the fish roe and process just until smooth.
With the machine running, drizzle in the lemon juice through the feed tube. Drop by drop, drizzle in the olive oil and process until it is completely incorporated. Remove the mixture to a bowl.
Gently fold in the creme fraîche and season with pepper. Refrigerate covered for 1 hour before serving.