Peel the beetroot and grate it using a box grater or food processor. Finely chop the onion and keep both vegetables ready before you begin cooking.

Heat the olive oil in a large saucepan over medium heat and add the chopped onion. Cook for 4–5 minutes until softened and translucent, stirring occasionally to prevent browning.

Stir the grated beetroot, mustard seeds, ground ginger, salt, and black pepper into the pan. Mix well so the spices are evenly distributed throughout the vegetables.

Pour in the apple cider vinegar and add the brown sugar. Stir until the sugar dissolves completely and the mixture begins to simmer gently.

Reduce the heat to low and cook for 35–40 minutes, stirring occasionally. The relish should thicken as the liquid reduces and the flavors develop.

Remove the relish from the heat and allow it to cool slightly. Spoon it into sterilized jars, seal tightly, and store in the refrigerator until ready to use.