In a food processor bowl, slice the avocados. Add 2 tablespoons lemon juice, 2 tablespoons oil, and ½ teaspoon sea salt flakes and process until smooth. Stir in the red onion. Divide the mousse among four serving verrines. Clean the food processor bowl.
Slice the smoked salmon into ribbons or cut it into small cubes. Top the avocado mixture with the salmon.
In a food processor bowl, combine 6 tablespoons yogurt and Boursin and blend until smooth. Note: You may use an immersion blender. Top each verrine with a layer of the yogurt mixture.
This can be eaten as is, or chilled before serving. Slide a breadstick (if using) into the side of each glass to garnish.