1/2 ripe avocado, peeled and pitted
1 1/2 tablespoons freshly-squeezed Mexican lime juice
1 1/2 tablespoons cider vinegar
2 tablespoons chopped fresh cilantro
1 clove garlic, chopped
2/3 cup vegetable oil
1 large egg yolk
Green top of 1 scallion, sliced
2 tablespoons cream
1/2 teaspoon salt

Combine avocado, lime juice, vinegar, cilantro, garlic and oil in a blender and process until smooth. Whisk the yolk in a bowl until broken up, then slowly whisk in the avocado mixture, a tablespoon at a time. Add the scallion and cream, then season with salt. Cover and refrigerate. If it becomes too thick during storage, whisk in a little more cream.