Preheat the oven to 350 degrees F. Combine the lettuce, onion and tomatoes in a large bowl; set aside.
Meanwhile, cut the bread into crouton-size cubes. Melt the butter in a skillet over medium heat. Add the bread cubes and toast, tossing, until golden, about 6 minutes. Set aside to cool.
Make the bacon: Mix the brown sugar, paprika, lemon zest and crushed pepper in a small bowl. Add the bacon and press to coat on both sides. Line a baking sheet with parchment paper; place the thyme sprigs on the parchment paper, then lay a strip of bacon on top of each sprig. Bake until the bacon is caramelized and crisp, about 20 minutes. Let cool slightly on the baking sheet. Cut the bacon into bite-size bits using scissors; add to the bowl with the lettuce. Discard the thyme.
Make the dressing: Whisk the mayonnaise, syrup, mustard and lemon juice in a medium bowl. Add the dressing to the salad and toss. (This is enough dressing for the whole salad, but you can use less if you want a less wet salad.) Sprinkle the croutons on top and eat.
Photograph by Kang Kim
Recipe courtesy Sunny Anderson for Food Network Magazine