For the gremolata breadcrumbs:
For the blue cheese-buttermilk dressing:
To assemble and serve:
Begin by pulsing the sourdough bread cubes in a food processor until fine. Place a large non-stick sauté pan over medium heat. Add olive oil and garlic and sauté until golden brown and aromatic, about 3 minutes. Add lemon zest and sauté very briefly, about 30 seconds, before adding breadcrumbs, parsley, and salt. Lower heat slightly and sauté stirring frequently until crispy, about 12 to15 minutes, being careful not to burn. Carefully transfer to sheet pan or large plate and spread out in thin layer to stop the cooking process.
Meanwhile, in a large mixing bowl combine the mayonnaise, buttermilk, sour cream, Worcestershire sauce, hot sauce, lemon juice, kosher salt, and black pepper, and whisk until well combined.
Pre-heat grill or cast-iron grill pan or skillet over high heat until very hot. Meanwhile, cut romaine lettuce heads in half, being careful to leave the core intact. Lightly brush or drizzle each lettuce half with canola oil, and season with kosher salt. Working in batches if necessary, quickly grill each lettuce half cut side down, applying light pressure, until blackened but still crisp, about 1 to 2 minutes.
Transfer grilled lettuce halves to platter or large plate and garnish with a few tablespoons of the blue cheese–buttermilk dressing, a few tablespoons of gremolata breadcrumbs, and all of the chives, shallots, and tomato. Serve immediately.
Any additional breadcrumbs or blue cheese-buttermilk dressing will keep covered in the refrigerator for up to 5 days.