In the bowl of a food processor, add the tuna and pulse to break up the pieces. Add the lemon juice, soy sauce, balsamic vinegar, and butter and puree. Scrape down the sides of the bowl and season with salt and pepper. Add the crème fraîche and puree the mixture until smooth. Transfer to a serving bowl and serve at room temperature. Can be kept in an airtight container in the refrigerator for 3 days. Allow to set at room temperature for about 30 minutes before serving. Enjoy!