Begin by preparing your cranberries. Sort through them, discarding any soft or blemished berries. Rinse the remaining cranberries under cold water. This step is crucial as it not only cleans the berries but also helps to wake up their pectin – the natural thickener that will give our sauce its jellied structure.

In a medium saucepan, combine the water and sugar. Bring to a gentle boil over medium heat, stirring until the sugar dissolves. I've learned that patience is key here; rushing the process can lead to a grainy texture as the sugar needs time to fully integrate into the water.

Add the cranberries to the saucepan along with the orange zest if using. Return to a boil, then reduce the heat to a simmer. The cranberries will begin to pop – a satisfying sound that signals they're releasing their pectin. Simmer for about 10 minutes, or until the cranberries have burst and the mixture has thickened slightly. I once simmered them for too long, and the pectin broke down, resulting in a sauce that never set. Watch for the telltale signs: the popping will slow down, and the mixture will coat the back of a spoon.

Remove the saucepan from the heat and stir in the freshly squeezed lemon juice. The lemon juice is not just for flavor; its acidity helps to balance the sweetness and ensures the pectin sets correctly.

Strain the cranberry mixture through a fine-mesh sieve into a bowl, pressing on the solids with the back of a spoon to extract as much liquid as possible. This is the moment of transformation, where the sauce goes from rustic to refined. I remember skipping this step in my early attempts, only to be met with a sauce that was more relish than jelly.

Pour the strained liquid into a clean saucepan or directly into a mold if you're feeling confident. If you're using a mold, lightly grease it with a neutral oil to ensure an easy release. I've used everything from vintage molds to simple loaf pans, and while the shape is a personal choice, the smooth, glistening finish is what makes this sauce a showstopper.

Let the sauce cool to room temperature, then cover and refrigerate. This is where the magic happens: the pectin chains will link up and set the sauce into a soft jelly. It needs at least 4 hours, but I prefer to leave it overnight. The first time I made this, I was too eager and cut into it too soon, resulting in a sauce that hadn't fully set and oozed rather than sliced.

When you're ready to serve, dip the mold in warm water for a few seconds to loosen the jelly, then invert it onto a serving plate. If it's stubborn, gently run a knife around the edge to encourage it. The sauce should slide out with a satisfying 'schloop' and hold its shape, ready to be sliced and enjoyed.