Preheat oven to 375F. Grease a baking dish. Bring water to a simmer in the lower part of a steamer.

Spiralize the zucchini on the widest ribbon setting, or slice very thinly. Sprinkle lightly with salt. Transfer the zucchini to the steamer basket, stretching the spirals to help the steam reach everywhere. Cover and steam until the zucchini is barely tender (about 5–7 minutes). Once cooked, remove the steamer basket and uncover. Place the steamer basket over a bowl to allow any excess moisture to drain.

Heat a large skillet over medium heat and then add the butter. Once the butter stops foaming, add the onions and cook until softened and beginning to brown on the edges. Stir in the garlic and cook for 1 minute.

Add the vinegar, stirring continuously and scraping up any browned bits, until the vinegar has almost completely evaporated. Pour in the cream and add the cream cheese in chunks. Continue stirring until the mixture is thickened and the cheese has melted.

Stir in the crab meat and green onions.

Stir in the Old Bay, salt, and pepper. Taste and adjust the seasonings, if necessary.

Add the steamed zucchini to the pan and mix everything together.

Transfer the mixture to the prepared baking dish and top with the shredded cheese.

Bake for 20–25 minutes, or until the casserole is bubbling and the cheese is melted and beginning to brown.

Let cool for 5–10 minutes before serving.