1. Heat peanut oil in a medium skillet over medium-high heat. Add the sliced onions and cook for 5 minutes, stirring occasionally. Add the diced apples and cook for 5 minutes or until apples and onions are pale golden brown. Season to taste with salt and pepper. Transfer to a small bowl
and let cool.
2. In a small bowl, whisk together the mustard and vinegar. Slowly drizzle in the olive oil. Season to taste with salt and pepper. Reserve. (Whisk again before re-using)
3. Toast crostini on both sides and while still warm, rub one surface with garlic clove. Place crostini on a baking sheet.
4. In the same skillet used for the onions, add shredded duck confit and warm over medium-low heat, about 1 minute. Drain any liquid from the onion mixture and add to skillet. Mix well to combine and continue to heat
5. Chop the frisée into bite-size pieces. In medium bowl, add frisée. Add enough vinaigrette to lightly coat.
6. To assemble: Place a portion of confit-onion mixture on each crostini, place a small mound of frisée on top of duck, and top with a portion of brie. Place baking sheet under the broiler to melt the cheese. Serve.