In a mixing bowl, combine garlic, egg yolks, mustard, lemon juice, horseradish, tarragon, salt, and pepper. Using a Vitamix Commercial immersion blender, combine ingredients until a paste-like consistency is achieved. Slowly stream in oil and continue to blend until mixture is fully emulsified. Transfer to an airtight container and refrigerate.
Heat oven to 450°F. In a mixing bowl, combine garlic, tarragon, and beef tallow. Set aside. On a work surface, pat beef bones dry. Using a knife, scrape and discard meat and fat off of the bone. Top bones with tallow mixture. Season with salt and pepper. Roast 10 minutes. Let cool.
In a mixing bowl, combine shallot, banana pepper, oil, mustard seeds, black pepper, and salt. Add beef and toss to coat. Set aside.
On a work surface, drizzle truffle oil over baguette slices. Season with salt and set aside. In a mixing bowl, combine parsley and tarragon. Set aside.
Place 1 piece Roasted Bone Marrow on a serving plate. Top with Beef Tartare. Garnish with radish and basil. Dot Beef Tartare with Garlic-Tarragon Aïoli. Serve with toasted baguette and parsley-tarragon salad.
