Heat oven to 350. Spray a 9" x 13" baking pan w/ cooking spray. In lg bowl, combine cornstarch, lemon juice and 1/4 cup cold water. Stir until cornstarch dissolves. Stir in sugar and berries, mix gently.
In another bowl, combine rolled oats, flour brown sugar, cinnamon and salt. Cut in butter until crumbly. Stir in peanut butter. Sprinkle mix over berries. Bake 20 mins.