Lightly butter an 20cm round springform tin. Mix 2 tablespoons Parmesan cheese with the breadcrumbs. Sprinkle this mixture into the pan; turn and tap pan to coat inside of tin. Wrap a large piece of foil underneath and up the sides of the pan. Be careful not to tear any holes in the foil.
In a medium frying pan, melt butter over medium heat. Add onions and green peppers and saute until tender, about 5 minutes. Remove from heat and let cool slightly.
In a large bowl, beat together cream cheese, eggs and cream until well blended. Fold in onions, peppers, salmon, Gruyere cheese, 3 tablespoons Parmesan cheese, salt and pepper. Pour into prepared tin.
Place the wrapped springform tin inside a large roasting tin. Place the tins in the oven and pour enough boiling water into the outside pan to come 5cm up the side of the springform tin.
Bake until firm to the touch, about 1 hour and 40 minutes. Carefully remove both tins from the oven and turn off the heat. Lift springform tin out of the water and return it to the oven. Let stand in the cooling oven for 1 hour. Transfer to a wire rack and let cool completely.