In a large bowl, combine butter lettuce, red onion, green onion, apples, blue cheese and walnuts.
Vinaigrette
In a shaker or sealable jar, add olive oil and sherry vinegar. Shake to combine.
Just before serving, shake vinaigrette and toss with salad.
Notes
Slice the apples thin to enjoy in each bite. If you're not serving immediately, toss the apples in citrus (lemon, lime or orange juice) to prevent browning.