Put diced potatoes into a pot and fill with cold water (water should just cover the potatoes). Put pot on stove on medium flame and add salt. When water comes to a boil, and the potatoes are cooked, drain the potatoes and spread out on a side towel to dry.
While potatoes are doing their thing, add bacon to a large saute pan and cook over a low/medium flame. When the bacon is cooked remove with a slotted spoon and drain the bacon fat, reserving some if you want to add it later. Wipe out the pan with a wad of paper towels (and crank up the heat), add a decent amount of a neutral oil, like canola, and heat it until you get some smoke.
Once the oil is hot, carefully add the potatoes and shake the pan a couple of times to make sure nothing sticks. Lower the heat a little and let the potatoes brown. Season with salt and pepper.
After a few minutes, shake the pan and get the other sides going. When the potatoes are almost completely cooked, add the red onion and let everything cook together. Once the onions are caramelized, add the capers, red wine vinegar and deglaze. When the vinegar is mostly evaporated, add the extra-virgin olive oil, mustard and bacon (and reserved bacon fat if you want), re-season with lots of cracked black pepper. Serve warm.