Put the tuna in a food processor and pulse to break up the fish. With the machine running, add the lemon juice, soy sauce and balsamic vinegar. Add the butter and blend until smooth, then stop the machine and scrape down the sides of the processor bowl. Season to taste with salt and pepper and blend again. Check the seasoning, then add the heavy cream and pulse to blend. Be careful not to over blend once the cream is added or the mixture may break.

Serve at room temperature, or cover and refrigerate for up to 4 days. Return to room temperature before serving.