Wash the cucumbers and green tomatoes and cut into 1/2"/1cm cubes. Peel and finely chop the onions.
Layer the vegetables in a colander placed over a bowl, lightly sprinkle each layer with salt, then cover and leave to drain for at least 6 hours, or overnight.
Discard the salty liquid and tip the salted vegetables into a large heavy pan. Reserve 2 Tbsp/30ml of the vinegar and add the rest to the pan with both sugars.
Slowly bring the vegetable mixture to the boil, stirring occasionally until the sugar has dissolved completely. Reduce the heat and cook, uncovered, for about 30 minutes, or until the vegetables are tender.
In a small bowl, blend the flour and mustard to a paste with the reserved vinegar. Stir the mixture into the relish and simmer for about 20 minutes, or until the mixture is very thick.
Spoon the relish into warmed sterilized jars, cover and seal.
Store in a cool, dark place for 1 week. Use the relish within 6 months. Once opened, keep in the refrigerator and use within 2 months.
I want to encourage youngsters (male or female) to get into the kitchen and enjoy cooking. TASTE it!