For the watercress oil, process watercress and olive oil in a small food processor until smooth. Strain through a very fine sieve.
Heat the oil in a large saucepan over medium-high heat. Cook onion, stirring, for 5 minutes or until softened. Stir in potato. Increase heat to high. Add stock and rosemary and bring to the boil. Reduce heat to low and simmer, partially covered, for 15-20 minutes or until potato is tender. Set aside to cool.
Blend until smooth. Return to medium-low heat. Add milk. Cook, stirring, for 3 minutes or until heated through. Add cheese and stir until smooth. Divide among bowls. Top with watercress oil, extra onion, watercress sprigs and bacon. Serve with toasts, if you like.
Serving size: 6
Recipe courtesy of taste.com.au