Special equipment: A large slow cooker (5 to 6 quarts)
Make the filling: Grease the insert of a large slow cooker (5 to 6 quarts) liberally with the 1 tablespoon softened butter. Place the peaches and berries in the insert and sprinkle with the lemon zest, juice, granulated sugar, flour and salt and toss to coat.
Make the topping: In a small food processor, process the pecans, flour, oats, granulated sugar, brown sugar, dried buttermilk powder (if using), cinnamon and salt until well blended. Add the butter and pulse just until the butter is pea-sized (about 5 pulses).
Sprinkle the topping over the fruit. Place a kitchen towel between the insert and the lid (to absorb condensation). Cook on high for 90 minute to 2 hours. Serve with vanilla ice cream.
Make the recipe in advance in the slow-cooker insert and pop into the slow cooker just before guests start arriving. This recipe works best on high for shorter periods of time. Long cooking times (over 4 hours) lead to the fruit losing some of its brightness of color and flavor. You may cook it on low for 3 to 4 hours if needed, adding a layer of peeled, sliced Granny Smith apples first, as they are sturdier and will protect the softer peaches and berries from the heat. Surprisingly, both frozen and fresh fruit work almost equally well (frozen was a tiny bit inferior in terms of texture and extra liquid it threw off).