Make the salad dressing: In a small bowl, whisk together the olive oil, lemon juice, Dijon mustard, honey, ½ teaspoon kosher salt, and pepper. Taste and adjust if needed (to make the dressing more zingy, add more lemon juice or mustard; to mellow it out, add more olive oil or honey).
Toast the walnuts: In a small skillet over medium heat, toast the walnuts for 4-5 minutes, stirring occasionally, until fragrant and turning darker brown in spots. Transfer to a small bowl and set aside to cool.
Using a sharp chef’s knife or a mandolin, slice the fennel bulb into very thin slices. Roughly chop up the fennel fronds and set aside.
In a large bowl, combine the walnuts, sliced fennel bulb, chopped romaine, apple, cucumbers, and green onions. Add enough dressing to lightly coat the lettuce and toss to combine (you might not need all of the dressing). Add the goat cheese and fennel fronds and gently toss once more. Add more salt and/or pepper to taste, and serve immediately.
Notes
This salad is best served immediately, but if you want to make it ahead of time, wait to chop the apple and dress the salad until you're ready to serve.