In a mixing bowl, whisk together yogurt and milk. Add flour and ½ cup sugar and mix until combined.
In a separate mixing bowl, toss together strawberries and rhubarb in remaining 1 cup of sugar until evenly coated.
Spread rhubarb and strawberry mix on the bottom of a 9"x13" glass baking dish. Pour batter evenly over top of rhubarb and strawberries.
Bake at 325ºF for 15-20 minutes, or until lightly brown on top. Remove from oven and brush top with melted butter. Return to oven and bake for an additional 5 minutes.
Remove from oven and let cool slightly before serving.
