Preheat oven to 375 degrees F.
Toss sliced rhubarb & strawberries in a bowl with the lemon juice, cinnamon & 1 tablespoon of sugar. Set aside.
In another bowl, combine the flour, 1 cup sugar, salt, baking powder & buttermilk. Stir just until everything is combined. Do not over stir.
Pour the melted butter in a 9x13 or similar size baking dish.
Pour the batter over the melted butter. Do not stir!
Pour the sliced fruits over the batter.
Bake uncovered for 40-60 minutes, depending on your oven & baking dish.