Place halibut in a baking dish only slightly larger than your fillet. Squeeze lemon juice over top and add salt and pepper. Garnish with lemon slices and drizzle wine over the surface of the fillet. Sprinkle chopped rosemary on top.
Bake halibut in preheated oven for 20-35 minutes until it flakes with a fork. I abhor over cooked fish, so I usually bake mine until the edges flake easily but the middle is only slightly firm. My thick fillet took 25 minutes.
Make the Vinaigrette:
Whisk together all ingredients through pepper. While continuously whisking, slowly add the olive oil until it is completely emulsified.
Store in sealed container in refrigerator. I think it actually got better overnight. Fact.