Prepare four small glass pots. Put a few watercress leaves into each of them.
Peel and dice the avocado, then toss them with some lemon juice to prevent them from browning. You can lightly mash them with a fork, if a more creamy textured preferred.
Cut the salmon into small pieces, then mix well with the mustard sauce. Season with ground pepper.
Divide half of the avocado pieces between the glasses, then spoon the salmon on top.
Top with the remaining avocado, the season with ground pepper and a squeeze of lemon juice.
Scatter some watercress leaves on top and serve.