Prep: Preheat the oven to 375°F. Spray a 2.5-3 QT. oven-safe baking dish with non-stick baking spray.
Make the filling: Dice large strawberries into quarters. In a large bowl, combine the berries, sugar, cornstarch, lemon juice, vanilla, cinnamon, and salt. Stir gently to combine.
7-8 cups mixed berries , 2/3 cup sugar, 3 Tbsp. cornstarch, 2 Tbsp. freshly sqeeuzed lemon juice, 1 tsp. vanilla extract, Pinch of cinnamon and pinch of salt
Add to baking dish: Pour into the prepared pan in an even layer.
Make the biscuit batter: Mix the flour, sugar, baking powder, and salt together in a mixing bowl. Cut in the cold butter - I like to work the butter into the flour mixture by pressing it between my fingers until the butter pieces are the size of peas. You can also use a pastry cutter or just two forks.
1½ cups all-purpose flour, 1/2 cup sugar, 1½ tsp. baking powder, 1/2 tsp. salt, 5 Tbsp. unsalted butter
Add buttermilk: Slowly stream in the buttermilk and stir until the dry ingredients are moistened.
2/3 cup buttermilk
Assemble: Scoop the dough onto the berry mixture using a cookie scoop.
Add sugar topping: Brush the tops with buttermilk and a sprinkle of coarse sugar.
Course sugar, 1 Tbsp. cold buttermilk
Bake: Bake for ~40-45 minutes Start Timeror until the tops are a nice golden/amber brown and the juices along the edges are bubbling.
Coll and enjoy: Allow to cool for ~10-20 minutes Start Timer, as the filling will thicken as it cools. Scoop and serve with vanilla ice cream or whipped cream.