Mash avocados lightly. Add lemon juice, onion and cilantro and mix well. Add spices and stir together. Place coulis in a plastic bag. Snip a tiny hole in 1 corner and pipe a spiral of sauce on top of the guacamole.
In a small bowl, mix water and cornstarch. Microwave on high for 15 seconds. Stir and microwave for 15 seconds more or until mixture is thick and clear. Puree all of the ingredients together.