1. Pat salmon dry and season with 1/4 tsp of the salt and the pepper. Heat 1 tbsp of the oil in a saute pan over medium-high heat. Place fillets skin side up in pan; cook 3 minutes. Flip and cook 3 more minutes for medium. Cool slightly on a plate, then cover with plastic wrap and refrigerate until cold.
2. In a large bowl, whisk remaining 2 tbsp oil, the lemon juice and zest, mustard and remaining 1/2 tsp salt. Remove salmon from fridge and flake into large pieces with a fork, discarding skin. Gently toss salmon in bowl with peas, watercress, mesclun and sunflower seeds.