Position oven rack to the highest level and heat oven to low broil.
Peel away the outer most leaves from the romaine head and save for other use. Pick out about 8-10 leaves, wash, dry and toss in olive oil, salt and pepper (amount as original recipe is written // adjust if altering batch size). Arrange in a single layer on a baking sheet and set aside.
Prepare dressing by adding mustard, vinegar, mayo, lemon juice, garlic, salt and pepper to a small bowl and whisking to combine. Slowly stream in olive oil (I usually go for the lesser end of the range) while continuing to whisk to emulsify the dressing. Whisk vigorously until well combined.
Add fresh rosemary, whisk once more and sample to taste. Adjust seasonings as needed, adding more salt, pepper or rosemary as desired.
Place romaine under broiler and watch closely while it cooks for 1-2 minutes. Flip when the leaves start to brown a bit, then cook for 1-2 minutes more. It’s a fairly quick process – you’re essentially looking for them to wilt a bit and soften.
Remove from oven, divide the romaine between serving plates and top each with 2 Tbsp dressing and several shavings of Parmesan cheese.
*Dressing (as original recipe is written) yields enough for ~ 6 salads. *Nutrition information is a rough estimate.