Special equipment: 3-inch ring mold
Combine the salmon, chives, cornichons, dill, mustard, onions and lemon zest in a medium bowl and season with salt. Chill until ready to use.
Preheat a grill pan to medium-high heat. Rub the baguette slices with the garlic, and then drizzle with olive oil and season with salt. Grill the bread until toasted and slightly charred.
Lightly dress the arugula with olive oil and season with salt.
Fill the ring mold with the tartar, and then remove the mold. Garnish with the dressed arugula on top and serve with crostini.