Heat the oil in a saute pan and gently cook the salmon, leaving the pieces slightly pink in the center, about 5 minutes. Cool. In a food processor or using two forks, shred the salmon and mix with the butter. Add the creme fraiche, shallot and tarragon. Season the mixture with salt, pepper and lemon juice. Pack into ramekins or similar small pots. Keep in the refrigerator, but before serving, take it out to soften because you want it nice and smearable.