Whisk together the lemon juice, mustard honey and salt. Continue whisking while adding the olive oil in a stream, blending well.
Transfer to a pitcher of jelly jar and reserve.
For the Baby Kale Salad:
Add the kale to a wide, shallow salad bowl. Top the greens with red onion slices, blue cheese, blueberries, and walnuts. Drizzle one-third of the lemon vinaigrette over the salad and toss well. Taste and add more vinaigrette if desired.
Alternatively, divide the kale onto four chilled salad plates, distribute the blue cheese, blueberries and walnuts evenly between the plates and drizzle with the vinaigrette.
Notes
The remaining salad dressing will keep, refrigerated in a covered container for 7-10 days.