Preheat the oven to 350 degrees. In a large bowl, mix together the peaches and the blueberries and pour into a baking dish. In a separate mixing bowl, mix together the sugar, flour and lemon zest. Whisk in the eggs and the heavy cream. Whisk in the salt. Pour the mixture over the peaches and blueberries. In another bowl, mix together the flour, brown sugar and walnuts. Mix in the melted butter. Sprinkle this mixture over the top of the baking dish. Place in the oven and bake for 25 to 30 minutes, or until the center is set, but still jiggles a little. Remove from the oven and cool. When fully cooled, place in the refrigerator for 4 hours.