How to Make It
Preheat broiler with oven rack 5 inches from heat. Spread tomatoes in a rimmed baking sheet or ovenproof skillet. Coat with cooking spray. Broil until tomatoes pop, 3 to 4 minutes.
Heat a grill pan over high. Coat with cooking spray. Grill onions until browned and tender, about 5 minutes per side. Transfer onions to a small bowl; cover with plastic wrap. Coat grill pan with cooking spray. Grill cut sides of lettuce until charred, 3 to 4 minutes per side.
Cook bacon in a medium skillet over medium until crispy, 6 to 8 minutes. Add shallot and thyme; cook, stirring occasionally, 2 minutes. Stir in vinegar; remove from heat. Whisk in oil and mustard.
Place 1 grilled lettuce wedge on each of 2 plates. Separate onion cross-sections into rings. Arrange tomatoes and onions over and around wedges. Drizzle with bacon dressing, and sprinkle with cheese and chives.