How to Make It
Preheat oven to 350°F. Toss together baguette pieces, 2 tablespoons of the olive oil, and 1/4 teaspoon of the salt. Bake in preheated oven until golden, 8 to 10 minutes.
Sprinkle garlic with 1/4 teaspoon of the salt. Let rest 2 minutes. Smash garlic and salt together using flat side of a knife to make a paste. Stir together egg yolk, lemon juice, mustard, and garlic paste in a medium bowl. Add vegetable oil in slow, steady stream, whisking constantly until combined. Add remaining 1/4 cup olive oil in a slow, steady stream, whisking constantly until mixture is thickened and golden yellow. Gently whisk in 2 tablespoons of grated bottarga.
Toss romaine and celery with 1/3 cup dressing. Top with croutons, parsley, black pepper, and remaining grated bottarga. Serve salad with remaining dressing.