Peel and quarter potatoes. Boil till potatoes are tender and can be easily pierced with fork, around 20 minutes.
While potatoes are boiling, fry bacon, drain on kitchen paper. You can also bake bacon in 200 C / Gas 6 oven for 20 minutes - this will be less messy. Line baking tray with foil. Afterwards drain on kitchen paper and chop.
Defrost frozen kale or spinach in microwave for 3 minutes (you can do it in original plastic package - just cut it open. Or cook on stovetop to defrost.) Drain and squeeze out some liquid. It doesn't have to be cooked at all. When I am in a rush, I just dump it frozen into potatoes, that cools the potatoes quickly too.
Mash potatoes with fork, add chopped bacon, 2 tablespoons vinegar, salt and pepper to taste. It is better to add salt at the end, since bacon can be quite salty too. Serve immediately.