Creamy cucumber soup

1 tablespoon extra-virgin olive oil
2 cloves garlic, minced
1 small onion, diced
1 tablespoon lemon juice
4 cups peeled, seeded and thinly sliced cucumbers, divided
1 1/2 cups vegetable broth, or reduced-sodium chicken broth
1/2 teaspoon salt
1/4 teaspoon freshly ground pepper
Pinch of cayenne pepper
1 avocado, diced
1/4 cup chopped fresh parsley [or dill], plus more for garnish