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, baking soda, and salt. Add in oats, brown sugar, and granulated sugar and whisk to blend, while using fingertips to break up small clumps of brown sugar.
Whisk vanilla into melted butter then pour melted butter over oat mixture. Stir with a spatula until mixture is evenly moistened.
Press 2/3 of the mixture into prepared baking dish, set aside remaining 1/3.
For the filling: Add diced strawberries and rhubarb to a medium mixing bowl. Toss with lemon juice.
In a small mixing bowl whisk together sugar and cornstarch. Pour sugar mixture over strawberry mixture and toss to coat.
Pour strawberry mixture over bottom crumb layer in baking dish. Evenly sprinkle remaining 1/3 crumb mixture over top.
Bake in preheated oven until top is golden brown and crisp and 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.
