1Place 2 tablespoons of the olive oil in a medium saucepan over medium heat. When the oil starts to shimmer, add leeks and thyme. Cook until leeks are soft, about 5 minutes. Add 2 tablespoons of the wine and cook until it evaporates. Add 2 tablespoons of the butter and the vegetable broth, and season well with salt and freshly ground black pepper. Bring to a simmer and cook, stirring occasionally, about 20 minutes. Using a slotted spoon, remove leeks to a plate and set aside.
2Use a small biscuit cutter (about 1 1/2 inches in diameter) or a shot glass to cut out bread circles. Cut as many rounds as possible from each slice (you need 24 total). Alternatively, you can cut the bread into 1-1/2-inch squares.
3Place a large frying pan over medium-high heat, add remaining olive oil and 3 tablespoons of the butter, and heat until foaming. Add mushrooms and sage, and stir to coat. Cook, stirring occasionally, about 5 minutes. Add remaining wine and cook until it evaporates. When mushrooms develop a crisp texture, about 5 minutes more, remove to a plate and set aside.
4In a small saucepan, melt remaining butter and set aside.
5Heat a medium nonstick frying pan over medium heat. Assemble a sandwich by putting a few pieces of Camembert and a teaspoon of mushroom mixture between two bread rounds. Close and brush melted butter on the outside of the sandwich. Repeat to make 12 sandwiches. Place sandwiches in the pan and cook until golden brown, about 1 minute per side. Remove to a plate.
6Garnish with a teaspoon of cooked leeks on top of each sandwich. Pierce with a bamboo pick or toothpick and serve.
Beverage Pairing:Mumm de Cramant Champagne NV, France. The major components of the dish—Camembert, mushrooms, brioche, and leeks—are together a call-out for Chardonnay. This Chardonnay, which happens to have bubbles and be from Champagne, is a great response. It has bright lemony sharpness to cut through the richness of the cheese, but also toasty, savory, and earthy notes to work with the rest of the dish.