1. Prepare Oven, Dish, and Fruit Filling:

• Preheat your oven to 400°F (200°C).
• Butter your baking dish (a 2-quart dish, such as an 8×8-inch square pan or 9-inch deep-dish pie plate).
• In a large bowl, place the 3 cups of blueberries and 3 cups of diced rhubarb. (If using frozen fruit, there’s no need to thaw it first).
• Sprinkle the fruit with the ¼ cup of granulated sugar and the 3 tablespoons of all-purpose flour.
• Toss gently but thoroughly until the fruit is evenly coated.
• Transfer this fruit filling mixture to the prepared buttered baking dish and spread it into an even layer. Set aside.

2. Make the Biscuit Dough Topping:

• In a separate medium bowl, whisk together the 1 cup of all-purpose flour, ¼ cup granulated sugar, ½ tablespoon baking powder, ½ teaspoon kosher salt, and ⅛ teaspoon baking soda until well combined.
• Add the cold, cubed unsalted butter to the dry ingredients.
• Using a pastry cutter, two knives in a crisscross motion, or your fingertips, quickly cut the butter into the flour mixture until it resembles coarse crumbs with some small, pea-sized pieces of butter still visible.
• Pour in the ½ cup of whole milk. Using a fork or a rubber spatula, stir gently just until a soft, sticky dough forms. Be very careful not to overmix; overmixing will make the biscuit topping tough.

3. Assemble and Bake the Cobbler:

• Drop large spoonfuls of the prepared sticky biscuit dough over the fruit filling in the baking dish. It’s okay if there are some gaps between the spoonfuls; they will puff and spread as they bake.
• Lightly brush the tops of the dough with a little extra milk.
• Sprinkle the 2 tablespoons of sanding or turbinado sugar evenly over the milk-brushed dough.
• Place the baking dish in the preheated oven. (Placing it on a larger baking sheet can help catch any potential drips).
• Bake for about 30 minutes, or until the fruit filling is hot and bubbling vigorously around the edges, and the biscuit topping is puffed and a beautiful golden brown.

4. Cool Slightly and Serve:

• Let the hot Blueberry Rhubarb Cobbler sit at room temperature to cool for at least 25-30 minutes before serving. This crucial step allows the bubbly fruit filling to set and thicken slightly.
• Serve the cobbler warm, scooped into bowls.
• It is absolutely delicious served with a generous scoop of vanilla ice cream, if desired. The cold ice cream melting into the warm, saucy cobbler is perfection! Enjoy!