Preheat and prep: Position a rack in the middle of the oven and preheat to 220°C / 430°F. High heat helps roast the fingerlings quickly so they develop deep golden edges without drying out. Rinse potatoes and cut any large ones so halves are uniform; leave skin on for texture. Pat salmon fillets very dry with paper towels — dry skin sears and crisps rather than steams.

Season potatoes: Toss 800 g halved fingerling potatoes with 4 tbsp extra-virgin olive oil, 4 tsp chopped rosemary, 1 tsp kosher salt, and 1/2 tsp freshly ground black pepper in a bowl. Spread cut-side down on a rimmed baking sheet in a single layer. Pressing cut-side down maximizes contact with the pan for browning.

Roast potatoes: Roast in the preheated 220°C oven for 20–25 minutes, shaking pan once halfway through. Toward the last 5 minutes, scatter 2 minced garlic cloves (optional) over potatoes and return to oven. High oven temperature and space on the sheet allow the potatoes to caramelize and get crisp edges while staying creamy inside.

Make glaze: In a small bowl whisk together 60 ml (4 tbsp) pure maple syrup, 20 g (4 tsp) Dijon mustard, 20 g (4 tsp) whole-grain mustard, 1/4 tsp salt and a generous grind of black pepper. The Dijon gives a smooth tang and the whole-grain provides texture; maple adds sweetness that caramelizes on the fish.

Prepare slaw: Thinly slice 1 full fennel bulb (≈240 g) and julienne 2 crisp apples (≈300 g). Toss in a bowl with 2 tbsp extra-virgin olive oil, 2 tsp lemon juice, a pinch of salt and pepper. Let the slaw sit at room temperature 10–15 minutes to allow the lemon to slightly soften the fennel and apple and for flavors to meld.

Char fennel (optional but recommended): Heat a large cast-iron or heavy skillet over high heat until smoking point. Add a drizzle (about 1 tsp) olive oil, then add sliced fennel in a single layer. Let sit without moving for 2–3 minutes until dark golden spots form (this adds smoky, caramelized notes). Remove from pan and combine with apple and dressing. Charring adds complexity and offsets the sweetness of the glaze.

Bring salmon to room temperature: While potatoes roast and slaw rests, bring salmon fillets to room temperature for 10–15 minutes — this promotes even cooking. Season skin lightly with salt (this helps dry the skin further and encourages crisping) and flesh with salt and pepper.

Sear salmon skin-side first: In a large nonstick or stainless-steel skillet, heat 4 tsp butter or neutral oil over medium-high until shimmering but not smoking. Add fillets skin-side down and press gently with a spatula for 20–30 seconds to ensure full skin contact with the pan; this prevents the skin from curling and ensures even crispness. Sear skin-side for 4–5 minutes depending on thickness until skin is deeply golden and releases easily.

Flip and glaze: Flip salmon carefully, spoon off excess fat if there's an accumulation, and immediately brush the maple‑mustard glaze over the flesh side. Cook for 1.5–3 minutes more (depending on thickness) until the salmon is just cooked through — glossy glaze will thicken and caramelize slightly. The short finish on the flesh side prevents overcooking and keeps the center moist.

Rest salmon: Transfer fillets to a warm plate, loosely tent with foil and let rest 3–4 minutes; carryover heat finishes cooking and resting lets juices redistribute. Taste potatoes and season to taste with more salt if needed.

Assemble plates: Lay a handful (1/2 cup) of baby arugula or watercress in the center of each plate. Arrange 200 g roasted fingerling potatoes beside the greens, place the glazed salmon fillet on top or next to the arugula, and spoon a generous portion of the fennel‑apple slaw alongside. Finish with a squeeze of 1 tsp lemon juice per serving over the salmon and slaw, and sprinkle 1 tbsp chopped fresh dill or parsley over each plate for bright herbal lift.

Final seasoning and serve: Add a final crack of black pepper and a light drizzle of olive oil if desired. Serve immediately so the salmon skin stays crisp and slaw keeps crunch.