Whole garlic cloves turn soft and mild when simmered or roasted slowly. Puree the soft cloves with butter and cream, spread it on the salmon, and bake until done to your liking. The dish couldn't be simpler or more aromatic. Note that the salmon should marinate for at least 30 minutes.
Remove papery outer shell from garlic, but leave bulbs intact and cloves unpeeled. Place bulbs in a saucepan just large enough to hold them. Add water to barely cover and bring to a simmer over high heat. Reduce heat to maintain a simmer and cook 40 minutes, adding more water as necessary to keep heads barely covered. Remove from heat, cool, and peel. Cloves will be very soft and easy to peel. Place cloves in a food processor with butter and puree. Add creme fraiche and mustard and process until blended. Transfer to a bowl and set aside.
Cut salmon into 8-ounce fillets. Spread top surface of each fillet with some of the garlic puree. Cover with plastic wrap and refrigerate for at least 30 minutes or up to 1 day.
Preheat oven to 350 degrees F. Place fillets on a lightly buttered baking sheet or in a buttered baking dish and roast until done to your liking (7 to 10 minutes). Serve immediately.