In a saucepan, heat the oil. Add the onion and cook over moderate heat until softened. Add the tamarind, garlic, chipotles, raisins, tomato paste, agave and chile powder. Bring to a boil and simmer over low heat until thick, 20 minutes. Let cool, then puree in a blender. Season with salt and pepper.
Set the salmon in a large, shallow dish and season with salt and pepper. Coat with 1/2 cup of the sauce; refrigerate for 1 hour.
Light a grill and oil the grates. Grill the salmon over high heat, turning once and basting with another 1/2 cup of the sauce, until just cooked through, 4 minutes.
For each club, use 3 slices of toast; spread each with 1 teaspoon of mayonnaise and layer with the arugula, tomato and fish.