Preheat the oven to 375F. Wrap the sweet potatoes in foil, leaving a small opening at the top, and place them in the oven. Cook until fork tender, about 1 ½ hours. When the potatoes are still warm, use your hands to peel off the skin. Cube the sweet potatoes and set aside.
Coat the bottom of a pot with oil and heat over medium heat. Add the onions and leeks. Season with salt and black pepper. Sauté for a few minutes until tender. Add the stock, thyme, and bay leaf. Bring to a boil and reduce to a simmer. Simmer for 10 minutes. Add the sweet potatoes into the pot and heat through. Transfer the soup to a blender and blend until smooth. Gradually add the milk and cream, adjusting the quantities to your desired consistency. Season with salt and black pepper to taste.
While the soup is cooking, coat a sauté pan with oil and heat over medium heat. Add the leeks and sauté until caramelized and slightly crispy, about 15-20 minutes. Season with salt and black pepper.
Serve a ladle of soup in a bowl and top with the leeks. Finish with a few cracks of black pepper and a drizzle of olive oil.