Bring a large pot of salted water to a rolling boil. Cook the dry pasta al dente, about 1–2 minutes less than the package says. Reserve ½ - 1 cup of pasta water, then drain and set aside.

In a large skillet, melt butter (or use a splash of olive oil) over medium heat. Add your sliced leeks and a pinch of salt. Cook gently until soft and fragrant, about 5–7 minutes.

Stir in the minced garlic and cook for 30 seconds until fragrant.

Add the horseradish and mustard, then pour in the wine to deglaze the pan. Let it simmer and reduce for 1–2 minutes.

Lower the heat. Add the sour cream and heavy cream, stirring until smooth and just warmed through; don't let it boil.

Reduce the heat to low and stir in the shredded Gouda and Gruyère, a handful at a time. Let each addition melt before adding the next. Stir until smooth. If it looks too thick, loosen it with a splash of reserved pasta water.

Add the drained penne to the skillet and toss to coat. Let it simmer together for 1–2 minutes to marry the flavors. Add more pasta water if needed.

Gently flake the hot smoked salmon into bite-sized pieces and stir it into the sauce. Don’t overmix, you want distinct, tender pieces. If you're using cold-smoked salmon, add it right at the end and warm it.

Turn off the heat and stir in plenty of fresh chopped dill and lemon zest. Give it a taste, add a squeeze of lemon juice or a pinch of salt if needed.

Spoon into bowls and finish with extra dill, a few small pieces of salmon on top, and maybe a final sprinkle of cheese if you're feeling extra.