Strawberry Rhubarb Mixture:

4 cups rhubarb
3 cups strawberries
1/2 plus 1/3 cup sugar
1/2 cup flour
1 tbsp. unsalted butter

Soak rhubarb in 1/2 cup of sugar for 1 hour cut into 1/2-inch pieces. Cut strawberries in quarters.

Preheat oven to 375°F.

Turn rhubarb into large bowl and add strawberries and remaining 1/3 cup sugar. Mix well. Add flour and mix again. If mixture remains too soupy, add more flour to thicken. Butter a 2-1/2 quart gratin dish with 1 tbsp. unsalted butter and pour in strawberry/rhubarb mixture.

Sprinkle with oatmeal topping (see below).

Oatmeal Topping:

3/4 cup all-purpose flour
8 tbsp. (1 stick) unsalted butter, cold, cut into small pieces
1 tsp. ground cinnamon
1/2 cup dark brown sugar
3/4 cups rolled oats

In food processor, combine flour butter and cinnamon. Pulse until large crumbs form, about 20 seconds (if you don't have a food processor, rub butter into flour and cinnamon until large crumbs form). Pour into a bowl and add oatmeal and sugar. Crumble with fingers until combined well.

Bake on middle rack in oven for approximately 1 hour, until rhubarb is tender, juices are bubbling and topping is crisp.