Preheat oven to 350°F
Set your cooking fire (or stove) to medium-high and in a large skillet melt the butter for the veggies. Add in the garlic and cook for 1 minute
Pour in the onions and mushrooms. Cook for 3 minutes
Pour in the spinach and cook until wilted. You may have to work in batches to fit it in the skillet. Set veggies aside
In a saucepan, melt the butter for the cream sauce. Add the flour and stir for 1 minute. Add the nutmeg and Old Bay and stir for 1 minute.
Add the milk and bring to a boil. Reduce the heat of your cooking fire to low and let simmer for 2 minutes. Add salt and pepper to taste
Spray casserole dish with non-stick spray. Layer the potatoes, veggies, sauce, and fish in the casserole dish. Layers should be just enough to entirely cover the width of the layer but not stack.
Top with cheese and panko.
Bake for 1 hour and 30 minutes. If your dish is very wide and flat as opposed to taller, cook for 1 hour and 15 minutes instead. Let stand for 10 minutes and serve to your hungry party. Now get back to your adventure!