Place halibut fillets, skin side down in baking dish coated with cooking spray or oil. Squeeze a lemon over and sprinkle with a little course Kosher salt.
In a bowl, combine shredded potato which have been thawed a little in the microwave, olive oil, kosher salt, freshly ground black pepper, and minced shallots. Put mixture over the fish and dot with margarine. Bake at 425 °F for about 10 minutes.
Add grape tomatoes to the dish, and sprinkle again with Kosher salt and a little olive oil. Continue baking until fish is opaque at center. If the potatoes are browned before the fish is cooked, just cover the dish with foil. My fillets were pretty thick and took about a half hour. Absolutely delish.