Your guests will be begging for more brandade de morue! Attached is a New York Times article that includes a basic description of how to make it. It’s not exactly a recipe, so I will give you a few tips.
First, simmer the water, milk, and aromatics for 10 minutes or so to infuse the flavors, and then add the cod. Second, you can use the same cooking liquid to simmer the potatoes. The ratio of cod to potato is according to personal taste. We like more cod than potato, but if you want a milder fish flavor and a consistency like mashed potatoes use more potato. Do not process this in a Cuisinart, because you will end up with something gummy; I use a fork to mash everything together. Another tip: include the garlic from the cooking liquid in your mixture. You will add olive oil and cream to combine the ingredients, and again the ratio is to your taste. For a creamier brandade, use more (or all) cream; if you want a healthier version, use more olive oil. It should not be dry, and it should be fluffy. Finally, use panko (Japanese-style bread crumbs) on top for extra crunch.