Peel and cut potatoes into large pieces.

Place potatoes in a medium saucepan with salted water.

Bring to a boil, then reduce heat and simmer for about 15 minutes until potatoes are tender.

Drain the potatoes and return them to the saucepan.

Add 1 teaspoon of salt substitute and 1/4 teaspoon of black pepper.

Mash the potatoes over very low heat.

Gradually incorporate the heavy cream and 4 tablespoons of unsalted butter into the mashed potatoes.

Cover and set aside the mashed potatoes.

In a Dutch oven, melt the remaining 2 tablespoons of butter over moderately low heat.

Add the sliced onions and cook, stirring occasionally, until golden, about 10 minutes.

Add the minced garlic and cook, stirring, until fragrant, about 1 minute.

Stir in the sliced turnip or rutabaga, Brussels sprouts, carrots, chopped cabbage, thyme, remaining salt substitute and pepper.

Mix well to combine all vegetables.

Stir in the vegetable broth and bring to a simmer.

Cover and cook over moderate heat until the vegetables start to soften, about 5 to 10 minutes.

Uncover, increase the heat to moderately high, and cook until the vegetables are tender and almost no liquid remains, about 10 minutes longer.

Heat the broiler.

Transfer the cooked vegetables to a 9-inch pie plate.

Spread the mashed potatoes evenly over the top of the vegetables.

Broil until the potato topping is lightly browned, about 5 minutes.