Cover the raisins with the port and let sit overnight.
Heat the oven to 400°F. Cut off the bulb end of the squash; reserve for another use. Peel the cylinder end and cut into 1/2- to 3/4-inch dice. Thinly slice the red onion. In a bowl, combine the squash, red onion, olive oil, and maple syrup. Season with salt and pepper and toss to combine thoroughly. Spread the squash and onions on an oiled sheet pan and roast, stirring occasionally, until the squash is just cooked through and the squash and onions are browned, 20 to 25 minutes.
Cut the endive into 1/2-inch slices. Cut off the stem end of the frisée, separate the leaves, and cut them into smaller pieces. Core and roughly chop the radicchio. Combine the endive, frisée, radicchio, and spinach in a nonreactive mixing bowl; toss with 1/2 cup of the vinaigrette. Arrange the greens on six plates.
Heat the broiler. Slice the goat cheese into 6 rounds and arrange on a baking sheet. Broil until just soft, 2 to 3 minutes. Top each plate of greens with the still-warm squash and onions. Drain the raisins and sprinkle them, the toasted walnuts, and the bacon, if using, around the plate. Garnish with a round of warmed goat cheese. Drizzle a few teaspoons of the remaining vinaigrette around each plate and serve. Reserve extra vinaigrette for another use.