If the surface is in a plane without protruding or recessed sections (check this carefully), you can glue the plaster to the leca. It takes a lot of glue. At least 300 ml per section of 90x240cm. I recommend Superset. For me, it worked better than PL 400. Additionally, you need to find a way to fix the plaster while the glue dries (this is not entirely easy). If the glue doesn’t hold (can be heard when you tap on the plaster), you’ll need to cut down that section and glue it again. You’ll then need to fill the joint with spackle. Another method is to drill several pairs of holes in the plaster. In one, you press the glue through, and in the other, you check that the glue is really spreading behind the board.
The best and probably cheapest and simplest way is to frame (nail and glue) against the leca and then screw the plasterboard into the frames.
The downside is that it takes up a bit of space, but on the other hand, it allows for some electrical installation if you want a socket or switch on that wall.
The downside is that it takes up a bit of space, but on the other hand, it allows for some electrical installation if you want a socket or switch on that wall.