Prepare your veggies. Peel your cucumber if necessary (some cucumber types have a bitter flavor) and cut into dice- like size. Do the same with your clean tomato. Peel and cut your onion small too. Place all the ingredients into a bowl.
Kidney beans should be soaked previously for 10-12 hours and then cooked for 3-4 whistles in the pressure cooker. Strain from water and let cool. Mix into the vegetables.
Prepare the dressing by mixing the mustard paste with the salt, pepper and vinegar first. Then add the oil and mix it well. I like to use a shaker to blend the ingredients well together. You can double the vinaigrette dressing too if you want a more juicy salad.
Pour the vinaigrette over the mixed salad and serve cold.