Step 1

Preheat the oven to 350°F.

Step 2

Place the potatoes in a large pot and add water to cover. Bring to a boil over high heat. Reduce the heat to medium, cover the pot, and cook until the potatoes are tender when pierced with the tip of a sharp knife, 12 to 14 minutes. Remove the pot from the heat and drain off all but 2⁄3 cup of the cooking water. Use a masher to mash the potatoes well. Season with salt to taste, and set aside.

Step 3

Cook the onions and carrots in a large sauté pan over medium-high heat, stirring occasionally, until the onions are translucent and beginning to brown, 8 to 10 minutes. Add water 1 to 2 tablespoons at a time to keep the vegetables from sticking.

Step 4

Add the peas, corn, and broccoli. Cook until heated through, about 5 minutes.

Step 5

Meanwhile, combine the arrowroot powder with the plant milk in a medium bowl, and whisk until well blended. Add the mixture to the vegetables along with the nutritional yeast, and cook until thickened, about 5 minutes. Season with salt and pepper to taste.

Step 6

Transfer the vegetable mixture to a 9×13-inch pan. Spoon the mashed potatoes evenly over the top.

Step 7

Bake until bubbly and lightly browned, about 1 hour. Sprinkle with the chopped chives, if desired, and serve hot.