Preheat your oven to 375°F with a rack positioned in the center.

First, let's prepare the filling. Cut the rhubarb into ½-inch slices after trimming the stalks. Combine them with halved or sliced strawberries in a large mixing bowl. Add ½ cup of sugar, cornstarch, and lemon juice, then mix everything together thoroughly.

Pour this fruity mixture into a pie plate.

Now, onto the topping. In a medium bowl, mix together flour, oats, brown sugar, diced butter, and a pinch of salt. Use your fingers to blend the mixture until it resembles pea-sized pieces, ensuring no loose flour remains at the bottom.

Spread this topping evenly over the fruit in the pie plate.

To prevent any drips, place the pie plate on a rimmed baking sheet. Cover it with foil and bake for about 60 minutes. Then, remove the foil and continue baking until the topping turns golden brown, and the filling is bubbly throughout, which might take around 30 additional minutes.

Allow it to cool for at least 10 minutes before serving.