Step 1

Preheat the oven to 350 degrees F (175 degrees C). Grease an 8x8-inch baking dish with butter.

Step 2

Combine flour, baking soda, and salt in a bowl; stir to combine. Mix in oats, white sugar, and brown sugar until well combined, using the back of a spoon to break up any brown sugar clumps. Add melted butter and vanilla extract and stir well until mixture is evenly moistened.

Step 3

Press 2 cups of the mixture into the prepared baking dish, pressing down lightly. Set remaining mixture aside.

Step 4

Combine strawberries and rhubarb in a bowl. Drizzle with lemon juice and toss to combine.

Step 5

Mix together 1/3 cup sugar and cornstarch in a small bowl. Add to strawberry-rhubarb mixture and mix well. Pour fruit filling on top of the crumb layer. Crumble reserved crumb mixture on top of the fruit, spreading evenly.

Step 6

Bake in the preheated oven until the top has browned and the filling is bubbly, about 45 minutes. Remove and cool before cutting into bars.