Put the tuna in a food processor and pulse to break up the fish. With the machine running, add the butter, cream, balsamic vinegar, and soy sauce and blend until smooth. Then stop the machine and scrape down the sides of the processor bowl. Add the lemon juice and pulse again. Season, to taste, with salt and pepper and blend again. Check the seasoning.
Serve at room temperature, or cover and refrigerate for up to 4 days. If refrigerated, return the spuma to room temperature before serving.
Recipe courtesy of Michael Chiarello