Heat a medium saucepan over medium heat and toast the celery seeds for about 1 minute until fragrant.

Add the diced jalapeños, red onion, sugar, salt, black pepper, and white vinegar to the pan. Stir well to mix all ingredients.

Bring the mixture to a simmer and cook for about 10 minutes until some liquid has evaporated but the relish remains runny. Taste and add more salt or sugar if needed. If adding more, let it cook until it dissolves to remove graininess.

Remove the pan from heat, let the relish cool completely, then transfer it to a clean jar. Refrigerate and use within 2-3 weeks.