Preheat the oven to 375 degrees F. Butter a 2-quart baking dish and set aside.
In a large skillet, heat the olive oil over medium-high heat. Add the pancetta and cook, stirring, until golden brown, about 4 minutes. Add the onions and season with salt and pepper and cook, stirring, until the onions begin to caramelize, about 20 minutes. Add the thyme and garlic and cook 1 minute.
Bring a large pot of water to a boil. Add the Swiss chard and blanch until tender, about 3 minutes. Drain and squeeze on towels to remove excess liquid.
Spread a layer of potatoes in the bottom of the prepared dish, slightly overlapping the slices. Season with Essence, and top with a layer of the onions. Place a layer of Swiss chard on top, followed by more potatoes and onions. Repeat with remaining ingredients. Add the cream, top with cheese and cover with foil.
Roast until nearly all the liquid has been absorbed, about 1 hour to 1 hour and 15 minutes. Remove the foil and continue baking until golden brown and bubbly, an additional 15 to 20 minutes.
Remove from the oven and let sit for 10 minutes before serving.
Yield: 6 servings