Preheat the oven to 425°. In a coffee grinder, coarsely grind the coriander seeds with the mustard seeds and crushed red pepper. Season the salmon fillet with salt and black pepper and spread 1 tablespoon of the mustard evenly over the fillet. Press the ground spices into the mustard.
In a large nonstick ovenproof skillet, heat 1 teaspoon of the oil. Add the salmon fillet, mustard side down, and cook over high heat until lightly browned, 2 to 3 minutes. Carefully turn the salmon. Transfer the skillet to the oven and roast the salmon for 6 minutes or until cooked through. Transfer the fillet to a cutting board.
Meanwhile, in a medium bowl, whisk the lemon juice with the remaining 1 tablespoon of olive oil and 1/2 teaspoon of mustard. Add the watercress and parsley, season with salt and pepper and toss to coat. Cut the salmon fillet into 4 pieces and serve with the salad.