Make the topping first by combining the oats, flour, sugar, brown sugar, cinnamon and salt. Stir to combine.
Pour in the melted butter and stir to incorporate. Set aside.
In another bowl add the frozen berries, flour, sugar, salt, lemon juice and vanilla extract and toss gently to combine.
Pour mixture into an 8x8’’ or similar size casserole dish or pie plate. Glob the topping ingredients evenly over the top (I say “glob” because the mixture will be softer than your normal dry crumble topping, because of the melted butter).
Bake for 40-45 minutes until the topping is golden brown and berries are bubbling around the edges.
Remove from the oven, place on a wire rack, and allow to cool for at least 7-10 minutes before serving. Serve with a scoop of vanilla ice cream, if desired.
Cover leftovers and store in the refrigerator for up to 5 days.
*I use the Costco, Kirkland brand of frozen berries called “Three Berry Blend” that includes blueberries, blackberries and raspberries. *You can use fresh berries, but you may need to cut down on the baking time by at least 5 minutes.Make ahead instructions:To make ahead: Bake and cool the crisp completely, then cover it with a double layer of aluminum foil. I don't suggest preparing the crisp and storing it, unbaked, in the fridge because the topping will get soggy.To freeze: Freeze for up to 3 months. Thaw overnight in the refrigerator, then warm in a 350°F oven for 20-25 minutes or until heated through.