Slice the leek into 1/4-inch rounds, being sure to rinse between layers to remove any hidden soil. Chop the fresh dill and set aside. In a small skillet over medium heat, melt 1.5 tablespoons of butter and add the panko, stirring frequently until it's golden brown and fragrant, about 3-4 minutes. Transfer the toasted panko to a bowl and set aside—this gives you crispy texture that won't get soggy waiting for assembly.

Bring a large pot of salted water to a boil and add the fusilli, cooking for just 4 minutes—the pasta will finish cooking in the oven, so slightly undercooking it now prevents mushiness in the final dish. While the water comes to a boil, preheat your oven to 425°F.

While the pasta cooks, cut the salmon into 1-inch chunks, removing any skin. Heat a non-stick skillet over medium-high heat and add the salmon chunks without any fat—they'll release their own oils and cook in about 3-4 minutes, gently turning halfway through until just opaque. The salmon doesn't need to be fully cooked since it will finish in the oven. Transfer to a plate and set aside.

Drain the pasta from Step 2 and set aside. In the same pot, melt 3 tablespoons of butter over medium heat and add the sliced leeks from Step 1. Cook for about 5 minutes until softened and fragrant. Sprinkle the flour over the leeks and stir constantly for 1-2 minutes to cook out the raw flour taste—this creates your roux base for a silky sauce.

Gradually pour in the milk and vegetable stock, stirring constantly to prevent lumps from forming. Bring to a gentle boil, then add the mustard for a subtle tangy complexity. Remove from heat and stir in the fromage frais until completely smooth—I find adding it off the heat prevents it from breaking and keeps the sauce creamy. Season with white pepper and a pinch of nutmeg, stirring well. Taste and adjust seasonings as needed.

Add the cooked pasta from Step 2 and salmon chunks from Step 3 to the sauce in the pot, folding gently to combine without breaking apart the fish. Pour the mixture into a 9x13 inch baking dish and spread evenly. Top with the toasted panko from Step 1 and the fresh dill, pressing gently so the panko doesn't blow around in the oven. Bake at 425°F for about 15 minutes until the topping is golden and the sauce bubbles slightly at the edges.

Remove from the oven and let rest for 2-3 minutes before serving—this allows the sauce to set slightly and makes plating easier without everything sliding around on the plate. Serve directly from the baking dish or portion into bowls while still hot.