Chop 1/3 of the pancetta into small pieces. Place pancetta in a small sauté pan over medium heat. Stir and cook until the pancetta is crisp and golden. Pour off the fat and place the pan with the pancetta back on the fire. Add the butter to the pan. When the butter begins to foam, add the leeks with some salt. Lower the heat and cover. Cook for 3 minutes until the leeks soften, then add the butternut squash and thyme. Cover for 5 minutes, remove the cover and mix the squash, butter and leeks. Cover and continue to cook until the squash is fully cooked. Remove from the heat.
Beat the eggs and mix in the heavy cream and milk. Add a few turns of freshly ground pepper. Add the squash mixture and stir. Stir the entire mixture gently with a fork and taste for salt.
Pre-heat the oven to 325 degrees F with the fan on or 350 if you do not have a fan in your oven. Place the tart shells on a baking sheet and, using a large spoon, fill each shell evenly with the solid ingredients in the custard mix. Then spoon or pour the rest of the liquid left in the bowl into each tart shell.
Bake for 10 minutes then reduce the heat to 300. Cook for another 5-10 minutes until the top is lightly golden and the filling is domed in the middle. If you tap the side of the tart shell, the filling should move but not jiggle. Remove from the oven and serve warm or at room temperature.