Gradually pour in the oil, constantly whipping the dressing with a whisk.
Peel the avocado, remove the bones and cut into slices of medium thickness. Put the avocado slices on plates and immediately pour a small amount of dressing so that they do not darken. Salt and pepper. Put the lettuce leaves in a separate bowl, generously pour the dressing over them and mix gently. If necessary, add salt and pepper. Place the lettuce leaves on a plate on top of the avocado. Add grapefruit slices and garnish with coriander leaves. Serve with the remaining dressing.