In a small bowl, whisk the spring onion, lemon juice, mustard, and sea salt. Whisking, gradually add in the olive oil until it is emulsified into the dressing.
Toss the radishes with a pinch of sea salt and set aside.
Set half of the lettuce leaves in a large bowl and toss with 1 to 2 tablespoons of the prepared vinaigrette. Add the remaining leaves and toss with another 1 to 2 tablespoons vinaigrette.
Toss the radishes with the remaining vinaigrette. Transfer the lettuce to a bowl or serving platter and scatter it with the radishes and spring onions just before serving. Finish with a pinch of sea salt, ground black pepper, and any reserved onion flowers.