Preheat the oven to 375 degrees.

In a large saucepan, combine the rhubarb, strawberries, ⅓ cup brown sugar, 2 tablespoons water, and lemon juice.

Bring to a boil. Reduce heat to medium and cook for 4-5 minutes, stirring often, until rhubarb is soft.

Combine cornstarch and the remaining tablespoon of water. Gradually stir into the rhubarb mixture over medium heat. Bring to a boil, cooking and stirring for 2-3 minutes, until thickened. Remove from heat and set aside.

In a large bowl, combine the butter, oats, flour, and salt. Mix until crumbly.

Press ⅔ of the mixture into a greased 8 inch square baking dish. Bake at 375 for 10 minutes.

Remove from the oven and decrease the temperature to 350 degrees.

Spread rhubarb mixture onto the crust and sprinkle the remaining oat mixture on top.

Bake an additional 15-20 minutes, until golden brown. Allow to cool and then cut into 16 squares.