Preheat. Preheat your grill to 450-500 degrees F for high heat, direct grilling.
Make the ranch dressing. In a bowl, combine the sour cream and buttermilk, stirring until smooth. Add in all remaining ranch ingredients, stir, and set aside.
Cut. Cut your romaine heads into halves, slicing through the stem.
Dress the romaine. Using a basting brush, cover the cut side of your romaine halves in ranch, getting the dressing between the leaves as well.
Grill. Place the romaine halves, cut side down, on your grill grates, and cook over high, direct, heat until the ranch and romaine caramelize, but do not wilt. This only takes about 2-3 minutes. Do not close the lid of your grill at all while grilling the romaine.
Enjoy. Transfer your grilled romaine halves to a serving platter and top with blue cheese, bacon, and additional ranch, if desired.