Combine salt, mustard, thyme and pepper. Rub salmon fillets all over with honey and sprinkle with half the seasoning mixture.
Heat 2 tsps of the olive oil in a skillet over high heat. Sear both sides of salmon steaks. Set unwashed pan aside to re-use for grape sauce.
Toss spinach and garlic with remaining 1 tsp oil in a baking dish and spread over bottom. Place salmon on spinach, cover loosely with aluminum foil and bake at 300F (150c) for 10 minutes.
Saute grapes in skillet used to brown salmon. Add wine and bring to a boil, season to taste with remaining seasoning mixture; simmer until reduced by half. Serve salmon on spinach; topped with grape sauce.