YouTube is definitely your friend here. So many great woodworking channels. There's a woman who does this kind of project all the time, and unfortunately her channel name escapes me. If I can find it again, I'll add a comment for you.
Woodworking
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I second this. Try searching youtube for "slab flattening jig".
Here is what I came up with
I am interested in what you come up with for this. I have a router and a large table top I made, that due to my novice woodworking abilities, is not flat.
Why does it look like the millennium falcon?
Maybe this is a dumb question, but why not just use a hand plane?
Tearout would be my biggest concern. Hand planing a board with fairly straight grain? No biggie. Hand planing something like hickory with crazier grain? That can be a pain. I imagine a cross section like this would be difficult to manage.
I have used a hand plane to flatten coffee tables in the past, but I don't think it would work well with the circular grain pattern and it would take me a week. Plus walnut is hard as heck.
This looks to be end grain which takes a toll on a blade edge and would be extremely laborious with a hand plane.