Note: Preheat barbecue to medium.
In a bowl, mix shallot with mustard. Add olive oil and whisk to mix. Add vinegar and season to taste. Set aside.
Gently clean mushrooms with a damp cloth. Remove stems, keeping only the caps. Stems may be used for another recipe. Brush mushroom caps with a little dressing and cook them upside down on grill.
When mushrooms are tender, divide cheese slices between them so that interiors are filled. Let Canadian Brie melt and serve on a bed of lettuce, accompanied with remaining vinaigrette.
Recipe from Chef Véronique Gagnon-Lalanne.