Blend Avocado, hot water, lemon juice and optional green onion tops until very smooth.
With the blender running, add the olive oil in a slow stream (or add 1 Tbsp at a time, processing well between additions) to form a smooth, thick emulsion. If sauce is too thick, add a small amount of hot water.
Season to taste with salt, white pepper and cayenne.
Serve immediately, or hold in a hot (not simmering) water bath. This sauce is meant to be served only slightly warmer than room temperature. Reheat over hot water — sauce will break if heated too vigorously.