In a medium bowl, whisk together the heavy cream and mayonnaise until smooth. This helps the sauce come together evenly in the slow cooker and keeps it from separating.
Add the cream cheese cubes to the cream mixture and stir gently. It’s okay if the cream cheese doesn’t fully dissolve; it will melt as it cooks and create little pockets of creamy goodness.
Pour the cream, mayonnaise, and cream cheese mixture over the mackerel and celery in the slow cooker, making sure most of the fish is covered. Do not stir; leaving the layers helps the fish stay flaky as it cooks.
Sprinkle the shredded cheddar cheese evenly over the top. Again, don’t stir; the cheese will melt down into the sauce and form a delicious, bubbly top layer.
Cover the slow cooker with the lid and cook on LOW for 3–4 hours or on HIGH for 1.5–2 hours, until the sauce is hot and bubbly around the edges and the cheese on top is fully melted. Avoid lifting the lid too often so the mixture heats evenly.
Once cooked, gently stir from the edges toward the center to combine the layers, being careful not to completely mash the mackerel. Taste and adjust seasoning with a pinch of salt and pepper if desired (optional, but go light at first since the cheese and canned fish are already salty).
Let the mixture sit uncovered in the warm slow cooker for about 5–10 minutes to thicken slightly before serving. Spoon into bowls and enjoy as-is, or over steamed low carb veggies.
