To make the sharp sauce, melt the butter in small pot set over medium heat. Add the onions and cook until softened, about 5 minutes. Add the brown sugar and continue cooking until the onions are a rich brown colour, about 3 to 4 minutes. Add the flour and tomato paste and mix well. While stirring, slowly pour in the stock. Mix in the remaining sauce ingredients and simmer 4 to 5 minutes. Strain the sauce into another pot reheat a few minutes before the fish is cooked.
Preheat the oven to 400 degrees F. Set the fillets on parchment paper-lined baking sheet. Spread the mayonnaise/mustard mixture over the fish. Combine the breadcrumbs, parsley and cheese in a bowl; sprinkle over the fish. Bake 10 to 12 minutes, or until cooked through. Broil 1 to 2 minutes to brown the top. Transfer the fish to plates, sprinkle with chives or green onions, garnish with lemon, and serve.
Note: To make fresh bread crumbs, pulse 1 to 2 slices of white bread in a food processor until crumbly.