Preheat your oven to 375°F. In a large mixing bowl, combine 3 cups sliced strawberries with 2 cups diced rhubarb. Add 3/4 cup granulated sugar, 2 tablespoons flour, 1 teaspoon vanilla extract, and 1/2 teaspoon cinnamon. Toss gently to coat the fruit evenly and set aside. In a separate bowl, whisk together 1 1/2 cups flour, 1/3 cup sugar, 2 teaspoons baking powder, and 1/4 teaspoon salt. Cut in 6 tablespoons cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Pour in 3/4 cup milk and stir just until a soft dough forms. Do not overmix. Melt 2 tablespoons butter in a 12-inch cast iron skillet over medium heat. Pour the strawberry-rhubarb mixture into the skillet and cook for 3-4 minutes, stirring occasionally, until the fruit begins to release its juices. Remove from heat. Drop spoonfuls of the biscuit dough over the fruit filling, leaving small gaps for steam to escape. Sprinkle the top with 1 tablespoon sugar. Transfer the skillet to the preheated oven and bake for 35-40 minutes, until the biscuit topping is golden brown and the fruit filling is bubbling around the edges. Remove from oven and let cool for 10 minutes before serving. The filling will thicken as it cools.