Cook the pasta according to the package directions (10 minutes for my spaghetti). While it's cooking, make the sauce.
Heat the butter in a skillet over medium high heat, then add the garlic, sliced leek and salt. Cook, stirring, about 4 or 5 minutes, until the leeks are soft. Add the broth and lemon juice and simmer 2-3 minutes, until the liquid is reduced by half. Stir in the other Tbs of butter and turn the heat off.
Once the pasta is cooked and drained, put it in a bowl and add the sauce, tossing to mix. Garnish with the fresh parsley.