How to Make It
Place onion wedges, cut sides up, on a rimmed baking sheet. Drizzle evenly with 2 tablespoons vinegar and oil; sprinkle evenly with salt and pepper.
Preheat a gas grill to medium (350°F to 400°F) on one side, or push hot coals to one side of a charcoal grill. Place onion wedges, cut sides up, on oiled grates over unlit side of grill. Grill, covered, 20 minutes. Turn onions onto one cut side; grill, covered, until grill marks form, about 10 minutes. Turn onions onto remaining cut side; grill, covered, until tender, about 10 minutes. Remove from heat.
Let onions cool slightly; discard any outer charred pieces. Finely chop 2 onion wedges; stir together with softened blue cheese in a medium bowl until combined. Cut remaining onion wedges lengthwise into thinner wedges, and separate into petals. Spread blue cheese mixture evenly over baguette slices; top with onion petals. Whisk together honey and remaining 2 teaspoons vinegar; drizzle over crostini. Sprinkle with pine nuts; garnish with thyme leaves.