Heat a grill to hot, or turn on the broiler. Cook bell pepper until it is completely charred, turning so all sides cook evenly, 8 to 10 minutes. Transfer warm pepper to a small bowl, cover with plastic wrap, and set aside until cool.
Using your hands, rub off charred skin. Slice pepper in half, and remove seeds and stem. Cut pepper into diamond shapes that are slightly larger than a piece of baguette.
Using a wooden spoon, mix together butter and two mustards in a small bowl until well combined. Spread a generous dollop of mustard butter on each slice of bread.
Place remaining ingredients except marjoram in a medium bowl, and toss to combine. Place a pepper diamond on each piece of bread, and top with a mound of crab salad. Garnish each canape with several marjoram leaves, and serve.