Preheat oven to 250 degrees F. Remove skin from salmon. Cut both kinds of salmon into large cubes. Combine salmon, yolks, and creme fraiche in a food processor. Season with salt. With the motor running, pour the cream through the feed tube until you have a smooth paste that is light and airy. Divide salmon among four 6-ounce ramekins; place ramekins in a large roasting pan and fill pan with hot water about halfway up the sides of the ramekins. Cover the pan with aluminum foil and bake for 30 to 35 minutes until mousse springs back slightly when touched. Remove ramekins from water bath and cool on a wire rack for 10 minutes before placing in the refrigerator to chill for 30 minutes.

For the Pickled Onions and Cucumber, combine vinegar and sugar in a bowl until sugar has completely dissolved. Add onion and cucumbers. Cover and refrigerate for 15 minutes before serving. Drain and toss with parsley.

Serve mousse with Pickled Onions and Cucumbers and crispy toasted slices of baguette.