PREP THE SALMON: Rinse and dry the salmon filet; divide into four portions; sprinkle with 1/2 tsp dill weed, salt and pepper to taste; then dredge in the cornstarch and set aside for 15 minutes while you prepare the sauce.
PREPARE THE SAUCE: Heat a nonreactive saucepan over medium heat and add 1 tsp olive oil; when oil is hot, add the onion and garlic and saute until onion is translucent.
Add the mushrooms, dill weed, and thyme, and saute until mushrooms have released their liquid.
MEANWHILE: Preheat oven to 400 degrees F; Heat a heavy, oven-proof skillet (cast iron preferred) over medium high heat.
RETURN TO SAUCE: Combine the lemon juice and wine (and cornstarch if not using xanthan gum), and add to the mushroom mixture; Bring to a boil, then simmer on low heat for 5 minutes; Remove from heat and set aside.
SEAR THE SALMON: Place 1 tsp each olive oil and butter in the hot skillet; Swirl around to coat the bottom; Place the salmon filets in the skillet (skin side up), and sear for about 3 minutes, or until a nice brown crust has formed.
Carefully turn the filets over so the skin side is now down; Then set the skillet in the preheated oven and roast for about 8-10 minutes, or until center is cooked through; remove from oven and set aside to rest.
FINISH THE SAUCE: Place the mushroom wine mixture in a blender or food processor along with the 1/2 cup of half and half and xanthan gum; blend until smooth and thickened; microwave as needed to re-heat but do not allow to boil.
SERVE salmon with sauce on the side; garnish with fresh parsley, slices of lemon, and dill weed, if desired.