1. Mix mustard and mayonnaise in small bowl to blend. Season to taste with salt and pepper. DO AHEAD Can be made 3 days ahead. Cover and refrigerate.
2. Mix salmon and next 8 ingredients in medium bowl. Cover and chill until cold, at least 30 minutes. DO AHEAD Can be made 4 hours ahead. Keep refrigerated.
3. Spread 1 tablespoon mustard mixture on 1 side of each bread slice. Cut each bread slice into 6 pieces. Divide salmon tartare mixture among bread slices. Transfer to platter and serve.