How to Make It
In a large bowl, gently mix the ground chuck with 1 tablespoon of kosher salt, 1 teaspoon of pepper, 2 tablespoons plus 2 teaspoons of the harissa, the cumin, garlic powder and thyme. Form the meat into eight 3/4-inch-thick oval patties. In a medium bowl, toss the onion slices with the olive oil and season with salt and pepper.
Heat a grill pan. Cook the onions over moderate heat, turning once, until tender, about 10 minutes. Transfer to a plate and keep warm. Grill the burgers over moderately high heat, turning once, until medium, about 8 minutes. Grill the rolls until lightly toasted, about 2 minutes.
Meanwhile, mix the mayonnaise with the caraway and the remaining 2 tablespoons of harissa; season with salt and pepper.
Spread the mayo on the rolls. Top with the patties, onions, tomato and cucumber; close the sandwiches and serve.