Look for smaller acorn squash; they tend to be less fibrous than larger ones. Red miso is the most potent variety of miso and is a worthy investment for this recipe. In a pinch, lower-sodium soy sauce is a good substitute.
Preheat oven to 425°F. Cut squash in half vertically; scoop out and discard seeds. Cut squash into 12 wedges; toss with 1 1/2 tablespoons oil, fennel seeds, and 1/4 teaspoon salt. Arrange squash in a single layer on an aluminum foil–lined rimmed baking sheet coated with cooking spray. Roast at 425°F 10 minutes. Flip squash. Turn on broiler to high with rack in top third of oven. Broil squash until caramelized and tender, 6 to 7 minutes.
Meanwhile, whisk together syrup, miso, and pepper. Heat 1 tablespoon oil in a large nonstick skillet over medium-high. Add salmon, and cook until lightly browned, about 4 minutes. Flip salmon, brush with 1 tablespoon syrup mixture, and cook 2 minutes, brushing with 1 tablespoon syrup mixture. Remove salmon from skillet, and sprinkle with remaining 1/2 teaspoon salt. Toss watercress with lemon juice and remaining 1 1/2 teaspoons oil. Serve salmon with remaining syrup mixture, squash, and watercress.