Preheat the oven to 450 degrees F. Make the vinaigrette: In a small heavy saucepan, bring the wine, currants, shallots, vinegar and brown sugar to a simmer over medium heat. Reduce the heat to low and simmer gently until the currants are plump and the liquid is reduced to 1/4 cup, about 5 minutes. Remove the pan from the heat and whisk in the olive oil. Season to taste with salt and pepper. Transfer to a small bowl.
Prepare the vegetables: In a large bowl, toss the cauliflower with 2 tablespoons of the olive oil to coat. Season to taste with salt. Spread on a large rimmed baking sheet. Roast, stirring occasionally, until the cauliflower is tender and lightly browned, about 30 minutes. Remove from the oven. Reduce the oven temperature to 400 degrees F.
Meanwhile, heat a large ovenproof nonstick skillet over high heat. Add the remaining 2 tablespoons olive oil, then add the mushrooms and cook, without stirring, until golden on the bottom, about 4 minutes. Stir the mushrooms and cook until tender and well browned, about 4 minutes more.
Stir the roasted cauliflower, shallots and garlic into the mushrooms and cook until the shallots soften, about 1 minute. Stir in the sage. Season to taste with salt and pepper. Return the cauliflower mixture to the baking sheet and cover to keep warm. Wipe out the skillet.
Cook the salmon: Using a sharp knife, score the skin side of the salmon a few times. Season with salt and pepper. Heat the same skillet over medium-high heat. Add the olive oil, then place the salmon skin-side down in the skillet and cook 3 to 5 minutes, or until the skin is golden brown. Turn the salmon over; transfer the skillet to the oven and roast 3 to 5 minutes, or until the salmon is mostly opaque with a rosy center when flaked in the thickest part with the tip of a knife. Meanwhile, uncover the cauliflower mixture and rewarm in the oven if necessary.
Transfer the salmon to a platter or four dinner plates. Spoon the cauliflower mixture around the salmon and drizzle with some of the vinaigrette. Sprinkle with the pumpkin seeds, if using. Serve the remaining vinaigrette on the side.