Pre-heat the oven to 180 °C (350 °F).
Cut off the top of the head of garlic.
Place the bottom on a piece of aluminum foil and drizzle with oil. Pinch the foil closed to form a sealed packet.
Roast in the oven for 40 to 45 minutes. Remove from the oven and let cool.
Remove 3 or 4 cloves from the head of garlic. Store the rest in the fridge for future use.
Season the duck legs with salt and pepper.
In a large, high-sided oven-safe pan, heat the duck fat over medium heat. Brown the duck legs for 7 to 8 minutes per side. Transfer to a plate. Remove the excess grease from the pan, but keep 15 ml (1 tbsp).
Using the same pan, sauté the onion for 2 to 3 minutes.
Add the sherry and reduce by half.
Add the duck stock, thyme and bay leaf. Bring to a boil.
Add the previously browned duck legs. Arrange the roasted garlic cloves, cherry tomatoes and olives around the duck legs. Cover and roast in the oven for 1:45 to 2:15, until the meat easily separates from the bone. Gently remove the duck legs, tomatoes and olives from the pan and lay on a plate. Cover loosely with a sheet of aluminum foil and keep warm.
Return the pan to the stove. Bring the sauce to a boil, then reduce and simmer for 12 to 15 minutes over medium heat until the sauce thickens slightly.
Meanwhile, cook the tagliatelle al dente in a large pot of salted boiling water. Drain.
Add the butter to the same pot and melt over medium heat. Return the tagliatelle to the pot and add the oregano and thyme. Season with salt and pepper. Heat for 1 to 2 minutes, stirring.
Serve the duck legs with the sauce, tomato mixture and pasta.
