Preheat the oven to 325 degrees Fahrenheit, and spray 8 wells of a muffin tin with cooking spray (unless, of course, you have a silicone muffin mold, in which case no greasing is necessary). [I've also baked the batter in a single 7-inch silicone cake mold, which essentially negates the name "bouchon" but makes for a nice variation.].
In a medium bowl, break up the tuna with a fork, smashing it to a rough paste. Add the tomato paste, crème fraîche, eggs, gruyère, a good pinch of salt, Italian parsley, and onion, and mix well. The batter should be relatively smooth.
Spoon the batter evenly into 8 wells of the muffin tin, and bake for 20-25 minutes, until set and golden around the edges. [If you choose to use a 7-inch mold as mentioned above, the baking time will be longer; bake until the batter looks set and does not jiggle.].