Arrange a rack at center position and preheat the oven to 400 F.

In a large, heavy, ovenproof frying pan (preferably cast iron) set over medium heat, sauté the pancetta until golden and crisp, 5 to 7 minutes. Remove with a slotted spoon to drain on paper towels.

Remove all but 2 tablespoons of the drippings from the pan. (If you don’t have 2 tablespoons, add olive oil to make this amount.) When hot, add the potatoes and sauté, stirring, until they start to take on a little color, 5 to 7 minutes. Season with salt and pepper and place the frying pan in the oven. Roast for 15 minutes.

Remove the frying pan from the oven and add the mushrooms, garlic and the 3 tablespoons of olive oil. Add the dried thyme and season with more salt and pepper. Toss to coat well.

Return the pan to the oven and roast until the potatoes are golden brown and tender and the mushrooms are softened, about 15 minutes more. Remove the frying pan from the oven and stir in the reserved pancetta. If desired, garnish the center of the pan with fresh thyme sprigs. Serve warm.

If buying pancetta from a deli, ask for it to be cut thickly into 1⁄4-inch slices. Prepackaged pancetta is often thinly sliced.