Wash the cucumbers thoroughly under running water.
Peel the onion. Wash and core the peppers.
Chop or grate everything in the food processor or by hand into very small pieces - no pulp.
Boil water, vinegar, sugar, salt, and mustard seeds in a larger saucepan. Stir in the cucumber and pepper mixture. Let it simmer gently by a third without the lid. About 30 minutes. In between stir in the grated horseradish.
Heat at least 6 clean glasses (250 grams each with a twist-off lid) in a water bath.
Fill the clean, hot jars with the cucumber mixture, seal it tightly, and turn it upside down on the lid for a few minutes. Turn the jars the right way up and notice that the lids come in with a pop.
Keep the glasses cool.
The cucumber mix is suitable as an accompaniment to meat, meatballs, fish, mashed potatoes or directly on bread.
After a few days of rest, the cucumbers are well drawn and ideally suited for consumption.