For the crumb layer: Preheat oven to 350 degrees F. Butter a 9 by 9-inch baking dish and line with parchment paper, butter parchment*.
In a mixing bowl whisk together flour, oats, cinnamon, almonds, baking soda, brown sugar and granulated sugar.
Add melted butter to crumb mixture and stir until combined.
Press 2/3 of the mixture into the prepared baking dish, set aside the remaining 1/3.
For the filling: Add diced strawberries and rhubarb to a medium mixing bowl. Toss with lemon juice and lemon zest. Add sugar and cornstarch and stir to coat.
Pour strawberry mixture over bottom crumb layer in baking dish. Evenly sprinkle the remaining 1/3 crumb mixture over top.
Bake in preheated oven until top is golden brown and crisp and the filling is bubbling about 45 - 50 minutes.
Cool on a wire rack until warm then cut into squares. Serve warm with vanilla ice cream if desired.