Preheat the oven to 350 degrees F. Coat a 9-inch square, 2 1/4-inch-deep cake pan with cooking spray. Prepare the cake mix as directed; pour the batter into the pan. Bake 30 to 35 minutes. Cool slightly, then invert onto a rack; cool completely.
Meanwhile, make the "soup": Combine the strawberries, peaches, sugar and lemon juice in a large saucepan. Cook over medium heat until the fruit breaks down, about 15 minutes.
Puree the sauce with an immersion blender (or in a blender) until mostly smooth. Transfer to a bowl.
Prepare the pudding as directed; tint with yellow and red food coloring to resemble melted cheese.
Trim the top of the cake with a large serrated knife to make it level.
Cut the cake in half horizontally.
Preheat the broiler. Put the cake on a foil-lined baking sheet and brush the top with the melted butter. Broil until toasted, about 4 minutes.
Cut the cake in half diagonally. Spread the pudding on the bottom triangles, almost all the way to the edge.
Cover with the toasted top triangles, pressing gently so the pudding oozes out. Serve with the fruit sauce.
Photograph by Jeff Harris