Special equipment: 3-inch ring mold
Combine the salmon, capers, cilantro, Dijon, olive oil, chipotle puree and scallions in a medium bowl, and season with salt and black pepper. Chill until ready to use.
Preheat a grill pan to medium-high heat.
Rub the baguette slices with cumin, drizzle with olive oil and season with salt and black pepper. Grill until toasted and slightly charred.
Fill the ring mold with the tartar, and then remove the mold. Serve with the crostini.