Cut the rhubarb into 1-inch pieces and measure out your sugar and water. Combine the rhubarb, sugar, and water in a medium saucepan over medium-high heat, stirring occasionally until the mixture comes to a boil. This initial boil will help the sugar dissolve and begin breaking down the rhubarb's cellular structure, which is key to developing a smooth compote texture.

Reduce the heat to medium and simmer the rhubarb mixture for 10-15 minutes, stirring occasionally, until the rhubarb pieces are completely soft and begin to break apart. You'll notice the mixture becoming more jammy and the rhubarb losing its structure—this is exactly what you want. I like to gently press a few pieces against the side of the pan with my spoon to check for doneness; they should collapse easily.

Stir in the blueberries and lemon zest, then continue cooking for another 3-5 minutes until the blueberries have warmed through and released some of their juices, creating a thick, saucy consistency. The compote will thicken slightly as it cools, so don't worry if it looks a bit loose while still hot. I find that the blueberries add a subtle tartness that balances the rhubarb's acidity beautifully, so resist the urge to add more sugar.