1. Clean and prep the leeks: Trim off the dark green tops and root ends, keeping only the white and light green parts. Slice the leeks in half lengthwise, then into thin half-moons. Place the slices in a large bowl of cool water, swish to release any dirt, then lift the leeks out and drain well. Peel and dice the potatoes, and mince the garlic.

2. Soften the leeks: In a large heavy-bottomed pot or Dutch oven, melt the butter with the olive oil over medium heat. Add the leeks and a pinch of salt. Cook, stirring often, until the leeks are very soft, sweet, and translucent but not browned, about 8–10 minutes.

3. Add potatoes, garlic, and herbs: Stir in the minced garlic and cook for 30 seconds, just until fragrant. Add the diced potatoes, bay leaf, and thyme, tossing to coat everything in the buttery leeks.

4. Simmer the soup: Pour in the stock, adding enough to cover the potatoes by about an inch. Bring to a gentle boil, then reduce the heat to a simmer. Cook uncovered for 15–20 minutes, or until the potatoes are very tender and easily smashed with the back of a spoon.

5. Blend until smooth: Remove the bay leaf and any thyme stems. Use an immersion blender to purée the soup directly in the pot until completely smooth and velvety. If using a countertop blender, carefully blend in batches, venting the lid and covering with a towel to avoid steam splatters, then return the soup to the pot.

6. Finish with cream and seasoning: Stir in the heavy cream or half-and-half. Add the lemon juice if using, then season with the remaining salt and black pepper to taste. If the soup is too thick, thin with a little extra stock or water until it reaches your preferred consistency. Warm gently over low heat without boiling.

7. Garnish and serve: Ladle the soup into bowls and top each one with a drizzle of olive oil or melted butter, a sprinkle of chopped chives or parsley, and an extra crack of black pepper. Serve hot with crusty bread, croutons, or a simple green salad.