Butter inside of casserole baking dish. In small bowl, mix brown sugar, flour, cinnamon, and salt. In separate medium bowl, add peaches and raspberries.
Sprinkle dry ingredients over fruit, add lemon zest and juice, and gently toss to combine. Pour into baking dish and spread evenly.
Preheat oven to 350 and place cobbler toppings on top of prepared fruit, brush tops with heavy cream and sprinkle with coarse sugar.
Bake until tops are golden brown and fruit filling has gotten bubbly and thick, 25-35 minutes.
Cool and serve scooped generously into a bowl, with your choice of accompaniment.