In a food processor blend tomatoes until pureed. Place a fine mesh sieve lined with 2 layers of cheesecloth over a large container. Pour pureed tomatoes into the lined sieve and place in fridge overnight. Tomato water which has filtered through the cheesecloth will be a clear liquid.
Place tomato water in a saucepot and place over medium low heat to warm.
In a food processor, add prawns, fromage frais, whipping cream, and curry powder. Season with a little salt.
In a separate bowl, beat egg white until stiff. Fold in prawn mixture, cover, and refrigerate until ready to use.
Fill a saucepot with water and bring to a boil. Using 2 spoons, make small quenelles using the mousseline mixture. Allow to boil for about 1-2 minutes or until when pinched on both sides with fingers, mousseline mixture feels firm to the touch. Remove cooked quenelles and place on a plate lined with paper towel to absorb excess moisture. Finish mixture and refrigerate until ready to use.
Heat a large fry pan over high heat. Add oil and butter. When butter has melted add sliced lobster mushrooms and sauté until golden brown. Season to taste with salt and freshly ground white pepper. Keep warm.
In a saucepan over medium heat add vermouth and bring to a boil for about 2 minutes. Whisk in whipping cream. Slowly whisk in butter until sauce thickens slightly. Add chopped dill and lemon juice. Season to taste with salt. Keep warm.
Place halibut cheeks on a plate and season with salt and freshly ground white pepper. Heat a large fry pan over high heat and add oil. Gently place halibut in pan and sear cheeks for about 1 ½ to 2 minutes on each side. Remove from heat.
Place mousseline quenelles in sauce and place back on low heat until quenelles are warmed through, about 4 – 5 minutes. Reheat lobster mushrooms and tomato water. Place a small amount of lobster mushrooms in the centre of each bowl. Place halibut cheeks on mushrooms and a quenelle on top of fish. Pour in hot tomato water and serve immediately.