In a small saucepan, whisk together the champagne, orange juice and mustards. Bring to a boil for approximately 5 minutes, or until the mixture is reduced. Taste and add more mustard to suit your taste. Remove from heat and set aside to cool.
In a large bowl, toss together the salad greens, mandarin oranges, Brie and macadamias. Drizzle dressing on top and serve.