Prepare the fruit: Wash rhubarb stalks thoroughly and trim off all leaves (they’re toxic!) and ends. Cut into ½-inch pieces. Hull strawberries and quarter them, ensuring they’re roughly the same size as your rhubarb pieces for even cooking.
Combine dry ingredients: In a small bowl, mix together sugar, cornstarch, cinnamon, and salt until well combined. This prevents cornstarch clumps in your filling.
Mix everything in the pot: Add rhubarb, strawberries, and the sugar mixture to a heavy-bottomed saucepan. Stir well to coat all fruit pieces evenly with the sugar mixture.
Let it sit briefly: Allow the fruit to sit with the sugar mixture for about 5 minutes. This will draw out some juices and help prevent scorching when you begin cooking.
Cook the mixture: Place the saucepan over medium heat and add lemon juice. Stir frequently as the mixture comes to a simmer. You’ll notice the fruit beginning to release its juices.
Simmer until thickened: Once simmering, reduce heat to medium-low and continue cooking for 8-10 minutes, stirring constantly to prevent burning. The filling is ready when the rhubarb is tender but not falling apart and the mixture has thickened enough to coat the back of a spoon.
Add finishing touches: Remove from heat and stir in vanilla extract and butter (if using). The residual heat will melt the butter and incorporate the vanilla throughout the filling.
Cool before using: Allow the filling to cool completely if using in a baked pie. For a no-bake application, cool to room temperature, then refrigerate until needed.