Melt the butter in the pot over medium-high heat.
Add in the chopped vegetables and cook until tender, stirring occassionally, about 10 minutes.
Sprinkle flour over vegetables and stir constantly for about 2 minutes until the vegetables are all coated.
Add the stock, potatoes, bay leaf, salt, and pepper. Bring to a boil then reduce heat and simmer for 15-20 minutes or until the potatoes are soft.
Discard the bay leaf and stir in the milk and halibut.
Continue to simmer until halibut is warm, about 5 more minutes.
Serve warm from the pot with a side of sourdough bread or topped with shredded cheese.