1. Mix together the soy sauce, balsamic vinegar and lemon juice.
2. In a food processor or mortal and pestle (which I used) process or pound the tuna until it’s broken down but not smooth. Mix in the liquid seasonings and process or pound until the mixture is smooth. Add the chili powder.
3. Add the butter bit by bit, adding each cube only after the previous one has been incorporated. If using a food processor be careful not to overdo it as the heat from the machine can cause the butter to melt.
4. Add the cream or crème fraîche just until it’s incorporated.
Although he advises that this should be served near room temperature, I found it’s a bit more appealing served chilled. Either way, serve it along with small toasts or crackers, or a sliced up baguette. I could imagine it as an excellent spread in a sandwich along with some slices of tomato, chopped black olives, and a tangle of peppery arugula too.
The mousse can be refrigerated, well-wrapped, for up to three days.