How to Make It
Put the potatoes in a medium saucepan of salted water. Bring to a boil, reduce the heat, and simmer until the potatoes are tender, about 15 minutes. Drain the potatoes and put them back into the saucepan along with 1 teaspoon of the salt and 1/4 teaspoon of the pepper. Mash the potatoes over very low heat, gradually incorporating the cream and 4 tablespoons of the butter. Cover and set aside.
Meanwhile, in a Dutch oven, melt the remaining 2 tablespoons of butter over moderately low heat. Add the onions and cook, stirring occasionally, until golden, about 10 minutes. Add the garlic and cook, stirring, until fragrant, about 1 minute. Stir in the sliced mixed vegetables, carrots, thyme, and the remaining 1/2 teaspoon each of salt and pepper. Mix well.
Stir in the broth and bring to a simmer. Cook over moderate heat, covered, until the vegetables start to soften, 5 to 10 minutes. Uncover, increase the heat to moderately high, and cook until the vegetables are tender and almost no liquid remains in the pan, about 10 minutes longer.
Heat the broiler. Transfer the vegetables to a 9-inch pie plate, spread the potatoes over the top, and cook until lightly browned, about 5 minutes.