Heat an outdoor grill to medium-high (400° to 450°F/200° to 230°C).
In a large bowl, combine the chicken, 2 Tbs. of the harissa, 1 Tbs. of the olive oil and 2 tsp. salt. Using a Microplane, grate in the zest of the lemon, then halve it and squeeze in the juice from one half (reserve the other half for the sauce). Toss to coat.
Halve the zucchini crosswise, then slice into planks 1/4 inch (6 mm) thick. Add the zucchini on top of the chicken (yes, it can touch the raw chicken), drizzle with the remaining 1 Tbs. olive oil and season with 1/4 tsp. salt. Use your hands to coat the zucchini in oil and salt—it’s fine if some of the harissa gets on the zucchini, but you don’t want to totally toss the zucchini and chicken together.
Grill the chicken and zucchini, turning once, until an instant-read thermometer inserted into the chicken registers 165°F (74°C) and the zucchini is very, very tender—like, smushy, 5 to 7 minutes per side. It’s so good when it’s a bit smushy. Transfer everything from the grill to a large serving platter. Let the chicken rest.
Meanwhile, make the sauce: In a medium bowl, stir together the labneh, the remaining 1 tsp. harissa, the juice of the remaining lemon half and 1/4 tsp. salt. Taste and add more harissa or lemon juice if you want. Transfer to a small serving bowl.
Tear some soft herbs right over the top for a pop of color and freshness. Throw everything on the table and let everyone serve themselves! Serves 4.