In a 32 ounce jar, combine the honey and the vinegar. Warm them in the microwave for 30 seconds until they can easily be whisked together.
Add the mayonnaise, mustard, onion, parsley, and salt to the jar. Pulse with an immersion blender a few times to combine.
Slowly drizzle in the oil while mixing with the immersion blender. When the dressing is fully emulsified, taste the dressing and adjust the salt if desired. Cover with a lid and refrigerate until ready to serve.
Notes
If you do not have an immersion blender, this can be made in a standard blender as well. Just combine everything but the oil in the blender and then slowly drizzle in the oil while blending on low speed.