In a large bowl, mash the cold rajma nicely with hands, fork or vegetable masher. It will take a few minutes but avoid using a food processor or blender (it makes the beans sticky). Try to mash them as fine as possible. Little bits of skins here and there is okay. Separately mash the boiled potato as well. Don't grate the potato.
On medium heat, heat up oil in a pan (preferably non stick). Crackle the cumin seeds and immediately add ⅓ cup chopped onion, garlic and ginger all together to the pan. Saute for just 30 seconds and add the mashed beans and potato to the pan. Cook this mixture continuously stirring for 2-3 minutes, you can mash lightly as you go. It will start to clump up into a ball. But will be soft. Dont cook for long else kebabs will be dry.
Take off the stove and transfer to a bowl. Sprinkle all the powdered spices along with salt and rest of the onions, green chillies, cilantro and mint(if using).
Combine well gently mixing with spatula or your hand if needed. Knead for a few minutes. Taste and adjust the salt at this stage. Let the mixture sit for 10 minutes.
Oil your palms, divide into equal portions and make small patties with your hands. Smooth all around using your palms and fingers. You should get around 8 or 10. Make all the patties and place them on a plate before frying.
Brush 1 tablespoon oil on a cast iron skillet or a non stick pan. Once the skillet is hot (not super hot), place the kebabs on the skillet and let them fry for 3-5 minutes on a low to medium heat until the bottoms are darkish and crispy. You can add 1 teaspoon oil at intervals but don't add lot of oil all at once.
Carefully, using a wide spatula, flip and cook on the other side. These will be soft so be gentle. Cook till browned on other side. Switch off the stove and let the kababs rest on the skillet for 3-4 minutes. This sets them, if you pick up too soon, they will break.