1.35 kg small waxy potatoes
8 Tbs unsalted butter
60mL olive oil, plus more to serve
1–2 x 100 g jars capers, drained
1 large red onion, sliced into thickish wedges
Kosher salt and freshly ground black pepper
60mL apple cider vinegar, white wine vinegar or Champagne vinegar, plus more to taste
60mL whole-grain mustard
1 1/2–2 handfuls dill, coarsely chopped
Bring a large pot of well-salted water to the boil. Boil the potatoes until very tender, 12–15 minutes; drain and let cool slightly.
Meanwhile, melt the butter in a medium skillet over a medium-high heat. Cook, stirring occasionally, until it starts to foam and then brown, 2–3 minutes. Add the olive oil and capers and cook until browned and starting to crisp, 5–8 minutes. (Capers have a lot of moisture in them, frying them until crisp is getting rid of that moisture, so it can take a while.) Add the onions, season with salt and pepper and cook, tossing or stirring every so often, until the onions are tender and starting to brown at the edges and the capers are very crispy, 8–10 minutes.
Remove the pan from the heat, add the vinegar and toss to coat. Transfer the butter-onion mixture to a large bowl and add the mustard and dill.
Once the potatoes are cool enough to handle, crush them to expose their insides. (I like to do this in my palm, but you can also smash them on a cutting board.) Add them to the bowl and toss very well to coat, crushing the potatoes further as you go, seasoning again with salt and lots and lots of pepper – more pepper than you think. Adjust with more vinegar or a drizzle of olive oil if you think it needs it.