Pre-heat the oven to 375 degrees.
Place the berries in a colander and rinse with cold water. Drain well.
In a medium bowl, combine berries, lemon zest, lemon juice, cornstarch, and sugar. Gently mix until the berries are well-coated. Set filling aside while you prepare the topping.
In a medium bowl, combine the flour, oats, nuts, brown sugar, salt and cinnamon. Add the cold cubed butter to the mixture. You could use a stand-mixer with a paddle attachment to mix everything, but I don’t have one (and I’m guessing you might not either), so just work the butter into the dry ingredients with your hands until the mixture is crumbly. Don’t overwork it, clumps are a good thing! (Keep the topping in the refrigerator if you aren’t using it right away.)
Pour the berry filling into a 9-inch pie dish (or whatever dish/pan you happen to have close to that size).
Sprinkle the topping in an even layer over the filling.
Bake your crisp for 30 minutes until the top is golden brown and the juices are bubbling up around the sides. Serve warm. I obviously suggest getting some vanilla ice cream involved.