Slice the salmon lengthwise (against the grain) into 1 inch strips and then crossways, to cube. Slice the avocado into cubes. Thinly slice the beets and then cut slices into quarters.
Make the sauce; mix all of the ingredients together and set aside until ready to use.
Mix the salmon, avocado, beets, and watercress in a bowl. Top with salmon roe, sesame seeds and whatever other toppings you choose. Dress with the sauce just before eating.