Cut the potatoes into small medallions and place in pot. Boil for 10-15 minutes. Check if the potatoes are cooked by inserting a skewer. If it goes through, the potato is cooked. Strain the water from the potatoes, put them back in the pot and shake them around to rough up the outsides a little. Mash the potatoes a little, but ensure you leave some potatoes still in shape.
Slice the ham into small pieces and render/cook for 10 min on medium heat, then cool. Dice the cucumber and apples. Combine mayonnaise and milk in a separate bowl. Place the mashed potatoes, onion, apple, cucumber, grain mustard, and ham into a bowl and add the mayonnaise mixture; mix to combine. Check your seasoning adding salt and pepper to taste, then serve.