How to Make It
Combine egg yolks, vinegar, and Dijon in a blender, and process on high until frothy, about 15 seconds. With blender running, slowly drizzle in grapeseed oil until smooth and thickened, about 1 minute. Transfer to a medium bowl, and clean blender.
Add oysters and anchovies to blender; process until smooth, about 15 seconds, stopping to scrape down sides as needed. Stir anchovy mixture into egg yolk mixture until blended and smooth. Season to taste with salt. Chill until ready to use.
Preheat a gas grill to high (450°F to 550°F). Cut white part from leek, and reserve for another use. Wash green leek tops thoroughly, and pat dry with paper towels. Break spines of leek tops, opening them up so they can lie flat on grill grate. Place onion slice on oiled grate; grill, uncovered, until softened and charred, about 6 minutes per side. Let cool slightly. Arrange leek tops in a single layer on oiled grate; grill, uncovered, until slightly charred and crispy, 8 to 11 minutes, flipping often.
Finely chop charred onion to equal 1/4 cup. Thinly slice leeks crosswise, and finely chop. Stir together leek, onion, parsley, grapeseed oil, red wine vinegar, soy sauce, and honey in a small bowl until blended. Season to taste with salt.
Preheat oven to 350°F. Arrange rye bread slices in a single layer on a large rimmed baking sheet. Brush with olive oil, and sprinkle with salt to taste. Bake in preheated oven until golden brown and crisp, 22 to 25 minutes. Let cool slightly, about 5 minutes. Break bread slices in half.
Gently toss steak with 6 tablespoons charred leek gremolata, olive oil, and salt.
Spread a scant 1/4 cup oyster aioli on half of each of 6 plates; spoon tartare mixture evenly in a circle in center of each plate. Stand 4 rye toasts up in each tartare mound. Finish each with a drizzle of olive oil.