In a medium bowl, mash the cheese and mix in the lemon zest and chopped watercress.
Toast the bread. Spread 1/4 of the cheese mixture on each piece of toast. Halve the avocado lengthwise, carefully remove the pit, then use a paring knife to slice the flesh thinly in the shell. Use a soup spoon to scoop out the slices, and fan 1/4 of the avocado over each slice of toast. Top with radish slices and garnish with watercress sprigs.