Heat butter in a medium stockpot over medium-low heat.

Cook leek and red onion until soft, 6 to 8 minutes.

Season with 1 teaspoon salt.

Stir in potatoes and carrots.

Add stock and bring to a boil.

Reduce heat to low and simmer until potatoes are almost tender, 6 to 8 minutes.

Add salmon and simmer until opaque, about 3 minutes.

Stir in cream to heat through.

Stir in dill, 1/2 teaspoon salt, and 1/4 teaspoon white pepper.