Preheat the oven ot 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 and cinnamon. Stir gently to combine. Pour into the prepared pan in an even layer.
7-8 cups mixed berries (I used fresh strawberries, blackberries, raspberries and blueberries), 2/3 cup sugar, or less if preffered, 3 Tbsp. cornstarch, 2 Tbsp. lemon juice, 1 tsp. vanilla extract, pinch of cinnamon
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.
1 1/2 cups flour, 1/2 cup sugar, 1 1/2 tsp. baking powder, 1/2 tsp. salt, 5 Tbsp. unsalted butter, cold, cubed
Slowly stream in the buttermilk and stir until the dry ingredients are moistened. Scoop the dough onto the berry mixture using a cookie scoop. Brush the tops with buttermilk and a sprinkle of coarse sugar.
2/3 cup buttermilk, cold, for topping before baking: 1 Tbsp. cold buttermilk and coarse sugar
Bake for ~40-45 minutes or until the tops are a nice golden/amber brown and the juices along the edges are bubbling.
Allow to cool for ~5-10 minutes. Scoop and serve with vanilla ice cream or whipped cream.