Preheat oven to 350 degrees Fahrenheit.

Rinse and drain raspberries in a mesh strainer or colander. Place over a bowl to catch excess water. Set aside. (Rinse frozen raspberries until thawed and water runs mostly clear. Can gently press with a spatula to get excess water out.)

In a large bowl, mix together the crust/topping ingredients: butter, brown sugar, oats, flour, salt, and baking powder. Mix until there are no butter chunks and the mixture is starting to clump together.

Press 2/3 of the mixture on the bottom of a lightly greased 9x13-inch pan. (Optional: Line pan with parchment paper sticking out of the sides to easily lift the bars out of the pan after they are baked.)

In a separate bowl, mix together the berries, sugar, cornstarch and lemon juice. Evenly distribute the fruit mixture on top of the crust. Sprinkle the remaining crust mixture over the fruit layer.

Bake at 350˚F for 35-45 minutes or until crust browns and the filling bubbles and thickens in the center. Cover with foil if crust browns faster than filling bubbles in the middle.

For best results, CHILL 1-2 hours before cutting and serving. (May also eat warm like fruit crisp, but fruit center solidifies better when chilled.) Keep leftovers covered and stored in the refrigerator.