Leeks can often contain hidden dirt so to ensure clean leeks, place the sliced pieces into a colander and rinse well. Once clean, shake out all the excess water or allow to thoroughly drain in the colander for a few minutes.
Heat olive oil in a large skillet on the stove top over medium high heat. When the oil is hot add leeks and garlic and saute until softened. Stir frequently to keep the leeks from burning and if necessary reduce to medium heat.
When cooked, transfer to the crockpot along with the potatoes, vegetable stock and 4-6 sprigs of thyme. Cook for 6 hours on low heat or for 4 hours on high heat.
When the potatoes are tender. Transfer ⅔ of the soup to a blender and blend soup until smooth. Use a potato masher or the back of a spoon to coarsely mash the remaining ⅓ of the soup in the pan.