For the mashed potato topping:

• 2½ lbs Yukon Gold or Maris Piper potatoes, peeled and chopped
• ½ cup whole milk (plus more if needed)
• ¼ cup unsalted butter
• Salt and freshly ground black pepper, to taste

For the filling:

• 1 lb boneless white fish (cod, haddock, or pollock), cut into chunks
• ½ lb salmon fillet, skinned and cut into chunks
• 2 cups whole milk
• 1 bay leaf
• ½ small onion, peeled
• 3 tablespoons unsalted butter
• 3 tablespoons all-purpose flour
• ¾ cup chicken broth
• ½ teaspoon Dijon mustard
• ½ cup frozen peas (optional)
• 2 tablespoons chopped fresh parsley or chives
• Zest of ½ lemon (optional)

Optional topping:

• ½ cup shredded sharp cheddar cheese