Heat the oil in a sauté pan and gently cook the salmon, leaving the pieces slightly pink in the centre. Cool. In a food processor or using two forks, shred the salmon and mix with the butter. Add the crème fraîche, shallot, and tarragon. Season with salt, pepper, and lemon juice. Pack into ramekins or similar small pots. Keep this in the refrigerator, but take it out to soften before serving because you want it nice and smearable.