Chop off the root and and the dark green top of the leek. Cut it in half lengthwise then slice thinly into half moons. Place the pieces in a bowl of cold water and let them sit for 5 - 10 minutes without disturbing them. Carefully scoop the pieces out with a slotted spoon and drain on paper towels.
Heat the olive oil in a large soup pot over medium low heat. Add the leeks, garlic and herb sprigs (count them). Sweat until the leeks are very soft and the garlic is fragrant.
Pour in the box of stock and stir to combine everything. Bring to a boil. reduce the heat to low, cover and simmer for 45 minutes or so. The stock should smell wonderful and the herb stems should be bare of their leaves.
Remove the pot from the heat and fish out the herb stems. Make sure you have the same number of stems you put in (your immersion blender will thank you).
Using an immersion blender process the stock until it is smooth (how smooth is up to you and there will be some herb flecks)
Your "bulked up" stock is ready to use in your recipe.