Make the pickled radishes:

1. In a medium bowl, combine vinegar, water, sugar and salt.
2. Whisk to dissolve sugar and salt.
3. Add radish slices.
4. Cover and refrigerate at least one hour or up to 8 hours.

Make the avocado spread:

1. In a small bowl, mash avocado and lemon juice with a fork.
2. Add salt and pepper to taste.
3. Mix well.

Make the toast:

1. Toast bread. Spread an equal amount of avocado mixture on each slice of toast.
2. Top each slice of toast with radish slices, feta and herbs.
3. Serve immediately.
4. Extra pickled radishes will keep in the refrigerator for up to 2 weeks.