Step 1

Preheat the oven to 400 degrees F.

Step 2

Cut the squash in half lengthwise and remove the seeds and fibers. Cover with aluminum foil and place the halves, cut side up, in a foil-lined shallow baking pan. Bake for 40 to 50 minutes, or until easily pierced with a knife but still firm. When cool enough to handle, scoop out the pulp, leaving a firm 1/4- to 1/2-inch-thick shell all around. Mash the pulp and set side until needed.

Step 3

Meanwhile, combine the potatoes with enough water to cover in a large saucepan. Bring to a simmer, then simmer steadily, covered, until the potatoes are tender, about 10 to 15 minutes.

Step 4

Heat the oil in a medium skillet. Add the onion and carrot and sauté over medium heat until the onion is golden and the carrot is tender-crisp. Remove from the heat.

Step 5

When the potatoes are done, drain them and transfer to a mixing bowl. Add the milk  and mash until smooth. Stir in the onion-carrot mixture, followed by the peas, nutritional yeast, and nutmeg. Add the reserved squash pulp, and stir gently until the mashed potato and squash are well integrated.

Step 6

Divide the mixture evenly among the four squash shells. Bake for 15 minutes, until well heated through. Serve each half as one substantial portion, or cut each half crosswise to make 8 smaller portions.