PREHEAT OVEN TO 350F. Combine whitefish, lobster, onion, salt, brandy and tomato paste in a food processor and blend until smooth. Add the egg whites and blend until incorporated. Transfer the mixture to a bowl, cover and chill for 30 minutes. Stir in the cream until incorporated. Divide the mousse between buttered ramekins. Place the ramekins in a water bath, cover and place in oven for 20 minutes. To serve, turn the contents of each ramekin out onto a warm plate, spoon some Hollandaise-based sauce over each mousse and serve immediately.