Peel and finely chop the onion and garlic. Peel and finely chop the ginger. Cut passion fruit in half, scrape the flesh from the skin with a teaspoon. Halve lemons, squeeze juice and measure about 75 ml.
Place the prepared ingredients together with salt and 250 ml water in a pot, bring to the boil and simmer gently for about 10 minutes, stirring continuously. Pour the liquid through a fine sieve into a large bowl.
Stir in honey and let the liquid cool down.
Mix mustard seed, mustard flour and sugar, stir into the liquid. Fill mustard into clean glasses, chill and let it stand for at least 3 weeks.