As a rough first shot:
- Use a tool like Materialize to generate a pseudo-height map from
your diffuse image of the bread
- Having UV unwrapped your slice you can use the height map to bump-map the surface of the slice using a simple set of shader nodes something like this...
Here, if the height is less than a threshold, a fully transparent material is substituted for the gold, so as to poke holes in the bread. You might feel you need actually to displace the surface of the slice in shading, too, if your shot is close-up, or at a raking angle.
Once you've provided an HDR environment to reflect from the metal surface, you get this sort of result.
As for the metal itself, you could substitute this principled shader for something more sophisticated: from approximating physically accurate to quite convincing.