hugelkultur and clay

I haven't written for some time, but I have a problem in the garden.
Our soil is nasty clay with sand and the small concrete rubble we cannot sieve out.
I'm doing huegelculture beds, mainly because you can get all this organic matter for free and I hope to improve the soil over time. When I plant trees everything works well, because I mulch afterwards.
But when I plant those vegetables which I directly sow like carrots, lettuces and the like the top layer bakes. I put some mushroom compost in the top layer and some manure, but it still bakes. I have one bed with only mushroom compost as a top layer which is slightly better.
We're in cool climate Australia, but it still seems to be either too much rain or baking sun.
What would you do as a top layer? I cannot mulch carrots or lettuces.

I have buried wood beds in clay soil.  They hold moisture well but I still have to irrigate to start seeds, especially if the bed is new.

what are wood beads? Do you mean wood chips?

I mean "hugelkultur" but below ground level - a pit filled with logs, sticks, and other organic material, topped with soil.    I've dug rocks out of my kitchen garden and replaced them with logs and other organic material, which has terrifically improved the soil's ability to hold moisture.  Eventually the entire garden will be one large buried wood bed, with logs even under the paths.  The earthworms are loving it!   

Sorry, I've read wrong I read beads instead of beds, but I actually use the same technique. But I don't put wood underneath the pathes. One bigger path which has  a drainage pipe gets all the stones.