Prep and preheat: Position a rack in the upper third of the oven (about 6–8 cm / 2–3 in below the broiler element) and preheat to 230°C/450°F (or set to broil if your oven has only a broil setting). Line a large rimmed baking sheet with foil for easy cleanup and lightly oil it. Preheating the oven and using a hot broiler rack ensures rapid browning and crisping when you finish the dish.

Slice the potatoes and fennel very thin: Using a mandoline or a sharp knife, slice the new potatoes paper‑thin (2–3 mm) and the fennel into similar thickness. Why: thin slices cook quickly and expose more surface area to the heat so the edges turn golden and crisp rather than leaving raw centers.

Par‑cook the potato/fennel layer (shortcut for even crisping): Toss the sliced potatoes and fennel with 3 tbsp (45 ml) olive oil, 1 tsp garlic powder, 1 to 1 1/2 tsp kosher salt (adjust to taste), and 1/2 tsp freshly ground black pepper. Spread in a single, slightly overlapping layer on the prepared sheet pan — crowding is OK if slices overlap slightly; it helps form a cohesive crisped bed. Bake at 200°C/400°F (or in the preheated oven before switching to broil) for 18–22 minutes until the potatoes are mostly tender and edges are beginning to brown. Why: par‑cooking ensures the potatoes are tender through without burning under the broiler and gives the fennel time to soften and sweeten.

Prepare the citrus‑dill topping: While the potatoes roast, zest and juice the lemons (reserve about 1 tsp zest per fillet). In a small bowl combine the lemon zest, juice of both lemons, chopped dill (reserve a little for finishing), the rinsed capers (roughly chop if large), 1/4 tsp garlic powder, 1/4 tsp salt and a few grinds of black pepper. Whisk in the remaining 1 tbsp (15 ml) olive oil to make a loose dressing. This bright dressing both seasons the salmon and acts as a finishing vinaigrette for the potatoes — the acid will cut through the fish's richness.

Season the salmon: Pat each salmon fillet dry with paper towel (dry surface = better browning). Taste the dressing and reserve 2 tbsp for the potatoes; toss the rest with the salmon flesh side (not the skin) so the citrus, dill and capers adhere. Sprinkle each fillet lightly with salt and pepper and a touch of lemon zest. Why: oil and acid help carry the herbs and capers onto the fish and the dry surface encourages a golden exterior under the broiler.

Finish the potatoes & fennel under high heat: When the potato/fennel base is mostly tender and starting to brown, switch the oven to broil (or increase to 230–250°C / 450–480°F) and move the sheet pan to the upper rack. Arrange the salmon fillets, skin side down, directly on top of the potato/fennel bed — the potatoes will protect the skin from immediate scorching while the hot air crisps the exposed flesh. Why: placing the fish on the hot potatoes lets flavors mingle (lemon-dill juices seep into the potatoes) while the broiler quickly cooks the salmon without drying it.

Broil to doneness: Broil the salmon 6–9 minutes depending on thickness (for 160 g fillets about 6–7 minutes for medium, up to 9 minutes for well done). Look for opaque flesh that flakes easily with a fork and a lightly charred top. Important: start checking at 6 minutes — overcooked salmon becomes dry. Internal target temperature: 50–52°C (122–125°F) for medium‑rare, 55–60°C (131–140°F) for medium to well done. If you have an instant‑read thermometer, use it for precision.

Rest and finish: Remove the pan from the oven. Spoon any resting juices from the pan into the reserved lemon‑dill dressing and stir. Let salmon rest 2 minutes (resting lets carryover heat finish gently). If using, dot fillets with 1/4 tbsp butter each to add gloss and silk — it melts into the hot juices.

Plate: Using a spatula, transfer generous portions of the crispy potato and fennel bed to plates, top each with a broiled salmon fillet (skin side down or curl the skin up for presentation). Spoon the lemon‑dill‑caper vinaigrette over the fish and scatter remaining chopped dill and a few capers. Finish with a twist of black pepper and, if desired, a small drizzle (1 tsp) of extra virgin olive oil for shine. Why: the warm vinaigrette brightens the dish and the capers add bursts of briny contrast.

Serve immediately: Serve while the potatoes are still crisp and the salmon warm. Suggested accompaniments: a green salad or steamed greens dressed simply in lemon and olive oil to echo flavors.