Research has suggested that the omega-3s found in salmon help build muscle and burn calories. A diet rich in omega-3s may also trigger the "off button" on the fat-storing gene.
Preheat oven to 250°.
Spray a baking sheet with olive oil spray. Place salmon fillets evenly over baking sheet. Spray tops with olive oil (or brush with a couple tsp), and season with salt and pepper. Place in the oven and roast 20 minutes, until opaque and easily flakes with a fork. Remove from oven, and set aside to cool slightly.
While salmon is cooking, whisk together water, olive oil, mustard, maple syrup, scallions, and garlic in a small pot. Place on medium heat and bring to a quick simmer until thickened, about 1 minute. Season with salt and black pepper.
In a large bowl, toss together salad greens and endive. Chop or thinly slice apples, and toss in the bowl. If not eating immediately, toss apples with lemon juice. Divide salad among four plates. Top with salmon and walnuts. Drizzle with sauce.