Trim and chop rhubarb into 1/2-inch pieces. Measure sugar, water, and lemon juice.
In a medium saucepan combine rhubarb, sugar, water (or apple juice), lemon juice, and a pinch of salt. Bring to a gentle boil over medium heat, then reduce to a simmer and cook, stirring occasionally, until the rhubarb softens, about 10–12 minutes.
Mash a few pieces for a chunkier sauce or blend briefly with an immersion blender for a smooth consistency. If using cornstarch slurry, stir it in and simmer 1–2 minutes until thickened.
Stir in vanilla if desired. Taste and adjust sweetness or acidity. Cool to room temperature, then transfer to an airtight jar and refrigerate for up to 7–10 days or freeze for longer storage.
Notes
Frozen rhubarb can be used directly from the freezer; add a few extra minutes of cooking time.
Add strawberries in the last few minutes for a classic rhubarb-strawberry flavor.
Sauce will thicken as it cools; use cornstarch for a thicker, glossy finish.