Combine water, lemon juice, mustard, and oil in a saucepan and bring to a boil.
Season fillets with Tarragon and Mustard rub, salt and pepper.
Reduce temperature in pan so liquid is barely simmering. Gently place halibut in liquid, cover pan and simmer for 8-12 minutes, depending on thickness of fillets. Fish is done when it flakes easily with a fork. Remove with slotted spoon and serve with a drizzle of Tarragon Mustard Oil.
To make the Tarragon Mustard Oil, whisk together Butter Olive Oil, Tarragon and Mustard rub, lemon zest, and lemon juice until well mixed. Drizzle over halibut.