Like many of the others, I use cutting boards and trivets to protect the walnut countertops.
Our kitchen has both walnut and granite, so many times it's just easiest to put the hot stuff directly on the granite countertops and simply avoid the walnut, but we also "recycled" random scraps of granite from the original install several years ago to make trivets for on the walnut... Many countertop installers are happy to give away their small scrap from other jobs too...
Wooden feet with felt pads, bonded to the underside of any shape can be a complimentary look. They make nice cheese/Charcuterie boards too. example here is roughly 8"x12"...
hotpad1.jpg
PS. dull the edges of the broken granite by tapping with a small hammer (DAMHIKT )