In a small saucepan, melt about 1 tablespoon butter. Add shallot and garlic, cook for two minutes, or until softened and aromatic.
Whisk in remaining butter and dijon mustard until well incorporated.
Slowly add white wine, whisking as you go. Bring mixture to a simmer, simmer for 2 minutes.
Add dill, lemon zest, and heavy cream, whisking until smooth.
Bring mixture back up to a simmer.
As the sauce starts to simmer, place halibut in the oven, set the timer for 15 minutes (18 minutes if the fish is more than 1.5" thick)
Continue simmering the sauce as the fish cooks, stirring frequently as it reduces in volume.
Once the fish is done cooking - it’ll flake easily - Place the fish on your serving plates, top with sauce.
Serve immediately.