Heat oil in a skillet with tall sides over medium heat. Add the shallot, garlic, ginger and a pinch of salt and saute until tender, 3 mins.
Add the coconut milk, stock, fish sauce, coconut aminos, lime zest, lime juice, coconut sugar, thai chiles, coriander and whisk until well combined and bring to simmer.
Add the mushrooms, snow peas and halibut and cook, simmering, until the halibut is just cooked through and flakes easily with a fork, about 6 minutes.
Serve over steamed rice and garnish with scallions and cilantro.