1. Spread a thin layer of mustard on each piece of salmon. Mix paprika, garlic powder, onion powder, celery salt and brown sugar, and sprinkle over fish.
2. Broil on high on the middle rack for 10 minutes.
3. Sauté onion in oil until golden. Add split peas, salt and six and a half cups water. Bring to a boil, then reduce heat and simmer until soft, about one hour. Uncover and continue to cook until all the water is absorbed.
4. To assemble, spread a tablespoon of split pea purée onto each crostini. Top with a cube of salmon.