Combine fish, buttermilk, egg white, 1/4 teaspoon salt, and pepper in the container of an electric blender or food processor; process until well blended. Transfer mixture to a small bowl. Add the minced parsley, stirring well.
Place 1 heaping tablespoonful of the fish mixture in the center of each wonton skin. Lightly beat the egg yolk. Brush the edges of each wonton skin with the egg yolk; top with an additional wonton skin. Press the edges together with a fork to seal. (Reserve an remaining wonton skins for another use.) Place the filled wontons on wax paper, and cover with plastic wrap.
Place water in a Dutch oven; add 1 teaspoon salt, and bring to a boil. Add the wontons, 4 or 5 at a time, and return to a boil; reduce heat and simmer 5 minutes or until tender. Remove ravioli with a slotted spoon; place in individual serving bowls. Spoon Mustard-Tomato Sauce over each serving. Garnish with parsley, if desired.
For the sauce: Saute tomato in 1 tablespoon oil in a large skillet 3 minutes. Remove with a slotted spoon and set aside.
Add the remaining tablespoon oil to the skillet and add onion to skillet. Saute 2 to 3 minutes or until tender. Add the garlic, and saute an additional 2 minutes. Add the wine and water to the skillet; cook over medium heat until the liquid is reduced to about 1/2 cup.
Combine mustards in a small bowl. Add 1 tablespoon wine mixture, stirring well with a wire whisk. Add the mustard mixture to the wine mixture in the skillet; stir well. Add butter, stirring until the butter melts. Add the sauteed tomato, salt, and pepper. Cook over medium heat until thoroughly heated.