Preheat the oven to 375 degrees F (190 degrees C).
Combine flour, 1 cup sugar, and baking powder in a bowl; mix until evenly distributed. Add milk and whisk until batter is smooth with no clumps.
Combine blackberries, blueberries, peaches, rhubarb, remaining 1 cup sugar, and lemon juice in a medium pot over medium heat. Mix until sugar is evenly distributed. Increase heat to medium-high and bring to a boil.
Remove fruit mixture from the stove and pour into a 9x13-inch baking pan. Pour the batter over top and then the melted butter; do not mix. Top with walnuts and cinnamon.
Bake in the preheated oven for 45 minutes. Remove from the oven and let cool 5 to 10 minutes before serving.