Thinly slice spring onion horizontally with a mandolin and separate into rings. Heat oil in a small saucepan over medium-high heat until hot. Test oil temperature by dropping a spring onion ring into oil. If it sizzles and rises quickly to the top, the oil is ready. Fry spring onion until light golden and crisp (1-2 minutes), strain through a fine strainer into a heatproof bowl, transfer spring onion rings to absorbent paper to drain, set aside to cool. Reserve 60ml oil for dressing, cool.
For verjuice dressing, whisk together reserved oil, verjuice and vinegar in a bowl until well combined, season to taste.
Combine lettuce and chervil in a bowl, drizzle with verjuice dressing and toss to combine. Scatter with spring onion rings and serve immediately with remaining dressing.