Preheat the oven to 450 degrees F.
Place the acorn squash halves and onion on a baking sheet lined with aluminum foil and rub with the olive oil. Season with salt and pepper and roast until the squash is just tender and lightly browned, about 20 to 25 minutes. Remove the squash to a plate to cool and return the onion to the oven until golden brown and a toothpick inserted into the onion goes in easily, about 20 minutes longer. Remove and set aside to cool completely along with the squash. When cooled, remove the skin from the squash and cut into large dice. Cut the onion into julienne and reserve separately.
Cut each head of Bibb lettuce in half and place each half, cut side up, onto a large salad or a dinner plate. Press lightly on the lettuce to flatten and open up the leaves a bit. Divide the diced squash, julienned onion, black-eyed peas, pecans, and blue cheese evenly among the tops of the salads. Drizzle the warm vinaigrette over the tops of the salads, season lightly with salt and pepper, and serve immediately.
Combine all ingredients in a small saucepan and heat gently until sugar is melted, about 2 minutes. Serve warm over salad.