Slice cucumbers as thin as possible (a slicer/mandolin works great if you have one). Chop the dill and bell pepper. Cut tomato into wedges.

Put all vegetables in a bowl and mix with mustard, vinegar, salt and pepper. The vinegar might seem too strong at first but it will get milder after marinating. Add sugar if you like to make it milder.

Cover and let marinate in the fridge for 20-30 minutes. Taste and add more vinegar, salt, etc if needed. Great with sandwiches, bread, or a side dish with basically any meal.