Heat oil in a sauté pan over medium-low heat. Add leek and onion and cook for about 10 minutes. Add garlic and jalapeno and cook for an additional 30 minutes, until very soft.
(Alternatively, you can combine the vegetables and oil in a 9x13-inch pan. Cover and bake until you see them starting to brown and get soft.)
Once vegetables are cooled, mix in mayonnaise (feel free to add more in you like it creamier), lemon juice, salt, pepper, and sugar.