- To make the sharp sauce, melt the butter in a 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 to the pot, mix well and cook 1 minute more. While stirring, slowly pour in the stock. Mix in the remaining sauce ingredients, simmer 4 to 5 minutes, and then remove from the heat (see Note).
- Preheat the oven to 400˚F. Set the fish fillets on a 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 12 to 15 minutes, or until fish is cooked through and golden on top.
- When the fish is almost cooked, set the sauce back over the heat and return to a simmer. Add a bit more stock, if you find it too thick. When cooked, spoon some sauce on each of six plates. Now set a piece of fish on each plate, 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. For a smoother sauce, strain it through a fine sieve before reheating and serving with the fish.