Place the watercress or baby spinach in large bowl. Drizzle with balsamic vinegar and toss well. Divide among four small plates.
In large skillet, heat 2 inches water over medium heat until bubbles cover bottom and sides of pan. Crack eggs into small bowl (do not use eggs with broken yolks). When a few bubbles have broken surface of water, gently pour each egg into pan, leaving room between them.
Cook eggs, without stirring, until whites are just set and yolks are still runny, 2-3 minutes. Use rubber spatula to gently release eggs from bottom of pan, if necessary. Using slotted spoon, remove eggs from water and drain on paper towels.
Place poached egg in each avocado half. Coat medium skillet with olive oil spray. Heat skillet over medium heat and add prosciutto. Cook until crispy, about 2 minutes per side. Transfer two avocado-filled egg halves to each plate and serve immediately with two slices prosciutto.