Preheat oven to 375 degrees. Prepare a greased 8×8 baking dish.
Mix frozen berries, sugar, lemon juice, and salt in a pan. Cook over medium-high heat until boiling. Turn down to low; add the 3 tablespoons almond flour and stir for 3-4 more minutes. Let thicken. Transfer to an 8-inch square baking dish.
Stir together almond flour, oats, and salt. Add butter and brown sugar and stir dry until mixed. Using your hands, squeeze topping pieces together into clumps.
Sprinkle topping evenly over filling. Bake until bubbling in center and brown on top, about 45 minutes. Cool 30 minutes before serving.