How to Make It
Make the roll. In a medium mixing bowl, fold the smoked trout, crème fraîche, lemon juice, and zest, parsley, capers, and red onion together.
Place the slices of salmon, barely shingled over each other, on top of either a piece of plastic wrap—or, if you have it, a sushi/bamboo roller. This should result in a rectangular sheet of salmon.
Scoop the trout mixture onto the slices of salmon in a linear fashion from left to right, as when making a hosomaki roll.
Roll the salmon over the trout to make a tight pink roll, stuffed with trout. Refrigerate to firm the roll before you slice it.
Make the gribiche. In a blender, combine the mustard, hard-boiled egg, and oil together to make a thick, smooth egg sauce. If the mixture is too thick, just add a few drops of water and continue to blend. Season with salt, black pepper, lemon juice, and minced chervil to taste. (Don't blend the chervil or your gribiche will be green!)
Assemble the dish. Cut 12 pieces of marble rye into, ideally, the same size and shape of your salmon roll. Small cookie cutters or ramekins will do the trick if you don't have circle cutters.
In a pan or cast iron griddle, melt the cultured butter and toast the marble rye to dark golden brown on one side, but not the other.
Slice the salmon roll into 12 even cylinders and place them on the toasted bread, cut side up.
Spoon a small amount of the gribiche onto the cut side of the salmon roll.
Garnish with spoonfuls of pasteurized trout roe, a bit of dill, and some cracked black pepper.