There is no other word to describe it - "just yum". Totally lick-worthy!
Slit the leeks lengthwise and rinse them under running water. Don't miss this step. Leeks trap soil between their folds so it is essential to wash them thoroughly. Slice the white parts in 1/4 inch thickness. Reserve the dark green parts for soups or stocks.
In a medium size, deep skillet, heat the olive oil. Add the Leeks and Garlic. Cook over moderate heat, stirring occasionally, until softened and just beginning to brown, about 10 minutes. Add the Vegetable/Chicken Stock and Milk. Cook till the Leeks are tender. Transfer the mixture to a blender/Vitamix and puree until smooth. Season with salt and pepper.
Return the puree back to the skillet. Simmer the sauce. Add Cream ( or Milk again) and a little water to achieve the desired consistency. The sauce should be thick but spreadable for a dip and a bit more loose for mixing with pasta.
Turn off the heat and add Lemon juice. Transfer to a heatproof container and line the top with plastic wrap to avoid the formation of skin or use immediately.
Optional: Swirl in 2 Tablespoons of cubed cold Butter (off heat) for an extra rich sauce.
Storage: Keeps for 2 weeks in the refrigerator or 2 months in the freezer without cream. Add the cream when reheating it.
If freezing, bring it to room temperature before reheating. I like to take it out of the freezer and keep it overnight in the refrigerator before reheating it.