Heat grill or grill pan until hot. Bring 3/4 cup water to a boil in a medium saucepan. Add couscous and pinch of salt; stir. Cover, remove from heat, and let stand 5 minutes. Fluff couscous with a fork.
Combine lemon juice and zest, couscous, thyme, and mayonnaise in a bowl. Season with salt and pepper. Set aside.
Snap off and discard tough ends of asparagus. In a large bowl, combine asparagus, onion, and oil; season with salt and pepper.
Grill asparagus and onions until spears are slightly charred and rounds are soft and browned, about 10 minutes.
Spread one-fourth of couscous over each piece of flatbread. Place 4 asparagus spears, tips extending, over couscous. Place one-fourth of onions on asparagus; top with one-fourth of the lettuce. Roll each sandwich tightly; seal with a wooden skewer. Serve.