Add all ingredients for the filling to an 8 by 8-inch casserole dish or a 9-inch pie plate and toss to combine.
For the topping, use a fork to mix together the flaxseed meal, almond flour, erythritol, and salt in a small bowl. Add the coconut oil, lemon zest, vanilla, and stevia glycerite mix to combine. Stir in the walnuts.
Sprinkle the topping onto the filling.
Bake until the topping is light golden brown and the sauce from the berries is slightly thickened, about 30 to 40 minutes.