Preheat the oven to 350°F. Grease the bottom and sides of a 9" x 13" glass or metal baking dish. Line the bottom with a strip of parchment paper long enough to extend up over the short sides of the dish. Grease the parchment paper, too. (This makes the bars easy to remove.)

In a bowl, combine flour, brown sugar, rolled oats, baking powder, baking soda and cinnamon. Cut in butter and use your fingers to combine it completely until mixture is crumbly. Set aside about 1 1/4 cups of crumb mixture (this will be the topping). Press the remainder into the bottom of the pan, making sure it reaches all sides. Bake for 12-15 minutes, until golden brown. Remove and set pan on cooling rack.
In a clean bowl, whisk together sugar, lemon zest (about 1 tbsp, or less if you want to reduce the tartness), cinnamon and flour. Add raspberries, lemon juice and butter. Use your hands to gently combine until raspberries are evenly coated.
Spread raspberry filling over cooled crust. Sprinkle reserved crumbs over top. Bake at 350°F for 35-45 minutes, rotating pan every 15 minutes, until crust is golden and berries are bubbling.

Remove pan from oven and set on wire rack to cool completely before cutting into bars. Bars keep well in a container in the fridge for a few days. They also freeze well for longer storage.