Preheat the oven to 325 degrees. Lightly grease 8 wells of a standard-size muffin tin with nonstick cooking oil spray.
Place the tuna in a medium mixing bowl; use a fork to break up pieces any larger than a dime. Add the cheese, crème fraîche, tomato paste, eggs, onion, parsley, and salt, stirring to thoroughly combine. The mixture will be a soft orange-pink color.
Divide the mixture evenly among the 8 muffin wells. Bake for 20 to 25 minutes, or until the tops and edges of the bouchons are set.
Transfer the tin to a rack, and let cool for 5 minutes. Carefully run a small, thin knife around the edge of each bouchon to make sure it isn't stuck, then carefully remove them from the tin. They will collapse a bit as they cool. Serve warm or at room temperature.
Makes 8 bouchons, enough for 4 light eaters.