Heat the olive oil in a large (preferably, straight-sided) saute pan over medium-high heat. Add the pancetta and cook until golden brown and crispy, 4 to 5 minutes, stirring frequently. Reduce heat if necessary.
Add the unsalted butter to the pan, and add the sliced leeks. Saute the leeks for 6-8 minutes over medium-high heat until they are lightly caramelized and soft. Add the heavy cream, chopped thyme, and 1/4 cup of water to the pan, and stir ingredients together. Bring to a simmer and reduce the cream mixture for 2 to 3 minutes, or until thick enough to coat the back of a wooden spoon. Season to taste with salt and pepper.
Cook the pappardelle in a large pot of boiling salted water, stirring every few minutes, until al dente. Reserve 1/2 cup of pasta cooking water. Using tongs, transfer the pasta directly from the pot to the pan with the cream sauce (don’t worry if you transfer some pasta water along with it, as this will help thin the sauce and help it coat the pasta).
Turn off the heat. Stir the pasta gently in the sauce until it is evenly coated, add the grated cheese, and add pasta liquid as necessary. Serve immediately and garnish with fresh thyme leaves and grated cheese.