Cook bacon in a large stockpot over medium-high heat, turning occasionally, until evenly browned. This takes about 10 minutes. Drain on a paper towel-lined plate, crumble, and set aside. Reserve 1 tablespoon of bacon grease in the pot.

Add onion to the reserved bacon grease and stir over medium heat until tender, about 5 minutes.

Add diced potatoes to the pot and fill with just enough water to cover the potatoes. Bring to a boil and cook until the potatoes are almost tender, about 5 minutes.

Add the fish cubes, evaporated milk, whole milk, and butter to the pot. Bring to a boil, then reduce heat to low and let it simmer for 30 minutes. Season with salt and pepper.