Preheat oven to 350°F.
Place salmon fillets in a baking dish large enough to hold the salmon in one layer.
In a small saucepan over medium-high heat, bring the cider and honey to a boil and let the mixture bubble steadily until it reduces by half.
Pour the cider over the salmon, then let it sit for 10 minutes.
Set a large skillet with a heat-proof handle over medium-high heat. Sprinkle the flesh side of the salmon with salt and pepper. Set the fish in the pan. Cook for 2 minutes, brushing the top with cider glaze so that the fish begins to caramelize.
Turn the fillets over and brush with the remaining cider glaze. Add the lemon halves to the skillet. Transfer the skillet to the oven. Bake for 6 to 8 minutes or until the salmon flakes easily when tested with the tip of a knife.
While the salmon is cooking, in another large skillet over medium-high heat, melt the butter. Add the spinach, salt, and pepper. Cook for 1 minute, or just until the leaves begin to wilt. Pour the wine over them and continue cooking for 1 to 2 minutes more or until tender.
To serve, drain any excess liquid from the spinach and divide it among 4 plates. Arrange a piece of salmon on top and garnish with a lemon half.