For the dressing: In a small bowl, whisk together the vinegar, mustard salt and pepper. Next, slowly drizzle in the oil until smooth and emulsified.
For the salad: Peel and clean the exterior of the lettuce head, then from top to bottom, slice and quarter the lettuce. Next, over medium-high heat, add grapeseed oil to a saute pan and allow to warm. Sprinkle both sides of each medallion with the salt and pepper, and then add the medallions to the pan. Allow the medallion to cook, in batches if needed, for 2 to 3 minutes on the first side, and then flip and repeat the process. After cooking the second side, remove and allow the medallions to rest. To build the salad, lay the lettuce on plate, and then add the tomato, onion, blue cheese and 2 ounces of dressing and serve.