Remove the dark green parts and root end of the leeks. Cut them in half lengthwise. Leeks have to be cleaned really well. I put mine in a bowl of cold water and let them soak then rinse them off.
Finely slice the leeks
Add oil, butter, garlic, leeks, salt & pepper to the pot. Sauté on medium heat for about 7 minutes stirring occasionally.
Add cubed potatoes and thyme sprigs
Pour in broth, bring to a boil then reduce heat, cover and simmer for about 20 minutes or until potatoes are fork tender
Blend soup with an immersion blender or in a regular blender until silky smooth. Pour blended soup back into the pot and add cream If you desire