In a small dish, mix maple syrup and mustard. Coat the salmon fillets with this mixture in a shallow dish and marinate in the fridge for a few hours or overnight for the best flavour.
Let the fillets come to room temperature. Chop the bacon small and set aside. Heat the oil in a large pan over medium heat. Let the pan get very hot for at least 2 minutes. Place the fillets in the pan gently and cook for 4-5 minutes on each side, or until light pink throughout. Set aside.
In a clean pan, add 1-2 teaspoons of bacon fat. Add the onion and mushrooms and cook to desired consistency. Set aside.
To make the vinaigrette, place 3-4 tablespoons of bacon fat into a small saucepan. Whisk in the mustard, vinegar, and honey over low heat until combined and slightly bubbling.
Assemble the individual salads by placing spinach into bowls and topping with desired amounts of bacon, mushrooms, onion, and a salmon fillet. Drizzle warm vinaigrette over top and serve.