In a medium bowl, whisk mayonnaise, sour cream and buttermilk to combine. Add blue cheese, hot pepper sauce, salt and pepper and stir gently to combine without mashing up cheese. Cover and chill until using, or up to 4 days.
If you don't already have a fire or smoker going, fire up a charcoal or gas grill to medium heat with the lid closed.
Put lettuce in a large bowl. Separate onion rings and add to lettuce. Drizzle 1/4 cup of the vinaigrette over lettuce and onion. Season with salt and pepper and toss to coat lettuce with seasonings. Set aside to marinate.
Place tomatoes in a baking dish, keeping stems as intact as possible. Drizzle with oil and season with salt and pepper. Lay tomatoes on coolest part of grill and cook until softened and wilted, 8 to 10 minutes. Transfer tomatoes to a plate, taking care to keep them on stems, and spoon 1 tsp of the vinaigrette over them.
Remove lettuce and onion from marinade and lay cut side down on grill. Grill 2 to 3 minutes, until lettuce is slightly wilted and both lettuce and onion are charred in places.
To serve, arrange lettuce halves on a platter and spoon blue cheese dressing over them, using 1 to 1 1/2 tbsp per lettuce half. Lay tomatoes over lettuce and drizzle with remaining vinaigrette. Scatter blue cheese, bacon, onion and, if using, croutons over salad. Grind lots of black pepper over the top and serve.
Heat oven to 375°. Tear baguette into 3/4-inch chunks.
In a bowl, combine oil, garlic, salt and pepper. Add bread chunks and toss to coat with seasonings. Spread chunks in a single layer on a large baking sheet. Bake at 375° for 6 to 8 minutes, until edges are golden brown and crunchy. Allow croutons to cool to room temperature and use immediately, or store in an airtight container at room temperature up to 2 days.
Little Gem is a variety of romaine; if you can't find it, use baby romaine hearts.