For the dressing, in saucepan combine cranberries, sugar, and vinegar. Cook over medium heat, stirring often, 5 minutes or until cranberries soften and begin to pop. Remove from heat; cool.
Transfer cranberry mixture to blender. Blend on high until nearly smooth. Add juice and mustard; process to blend. With blender running, slowly pour in oil until slightly thickened and creamy. Transfer to bowl. Whisk in sage, salt, and pepper.
For Brie toast, preheat broiler. Place wedge of cheese on each toasted bread slice. Broil 5 or 4 inches from heat 1 to 2 minutes, until cheese is melted.
Toss spinach, dried cranberries, and seeds with 3/4 cup of the dressing. Serve with toast. Pass additional dressing. Serves 8.