Finely grate the radishes using a box grater or food processor, then transfer to a fine-mesh strainer lined with cheesecloth. Press gently to squeeze out excess liquid—you want the radishes moist but not dripping, as excess water will prevent the jelly from setting properly. While the radishes drain, sterilize your jars and lids by placing them in a 200°F oven for 5-10 minutes. I find it helpful to line a baking sheet with paper towels to catch any condensation as the jars cool slightly.

In a large, heavy-bottomed pot, combine the squeezed radishes, granulated sugar, and water. Heat over medium heat, stirring occasionally until the sugar completely dissolves and the mixture becomes clear. This typically takes 5-8 minutes. Once the sugar is fully dissolved, increase the heat to medium-high and bring the mixture to a rolling boil, stirring frequently to prevent scorching on the bottom.

Once the mixture is at a rolling boil, immediately stir in the fruit pectin, making sure to break up any lumps and distribute it evenly throughout the liquid. Continue boiling for exactly 1 minute, stirring constantly. The mixture should reach approximately 220°F on a candy thermometer—this is the gel point where the pectin will set properly. I always use a thermometer rather than relying on the cold plate test for jelly, as it's more reliable with pectin-based recipes.

Remove the pot from heat immediately after reaching 220°F. Using a skimmer or spoon, gently remove any foam that has formed on the surface—this ensures a clear, beautiful jelly. Quickly stir in the lemon juice, horseradish, and salt, distributing the horseradish evenly so its peppery bite is consistent throughout. Working quickly while the jelly is still hot, carefully pour it into the sterilized jars from Step 1, leaving about 1/4 inch of headspace. Wipe the jar rims clean with a damp cloth before sealing with the sterilized lids.

Place the filled and sealed jars into a water bath canner (or large pot) filled with enough water to cover the jars by 1-2 inches. Bring the water to a rolling boil and process the jars for 10 minutes. This ensures proper sealing and shelf stability. Carefully remove the jars using a jar lifter and place them on a clean kitchen towel to cool undisturbed. Let them rest at room temperature for 12-24 hours before checking the seals—you should hear a faint 'ping' as each jar cools and seals.