I made this soup once before for guests and it turned out something like green mashed potatoes. Don't let that stop you from making this soup -- because when done right, it is delicious. I was inspired by a daily soup from Little Nest Cafe a few months ago.
I don't have exact measurements, but it went something like this:
Break a bulb of garlic in half. Using only one half, cut the tips off the bulb. Wrap in aluminum foil, bake in 400°F oven for about 20 minutes. Slice 2 leeks lengthwise (white stalk only) then slice into approximately 1/2 inch pieces. Rinse in water to remove grit. Finely chop half a large sweet onion. In a large saucepan, at medium heat, melt 1 tablespoon butter and 1-2 tablespoons olive oil. Saute leeks and onion until mostly cooked through. Clean and peel 2 large potatoes and finely chop -- add to saucepan. Season with salt and pepper, cover and cook for a few minutes. Once potatoes have begun to soften, add 1 litre vegetable stock (or part stock, part milk). Stir it up. Once it comes to a boil, reduce to a simmer and leave (stirring now and then) for another 10 minutes. Remove your garlic from the oven and once it is cool enough to handle squeeze the cloves into your soup. Add a package of arugula/rocket and pulse it down with an immersion blender. Top with a drizzle of olive oil and toasted pine nuts and serve with toasted bread.