What additional testing is recommended with heterozygous HFE C282Y mutation whose ferritin is elevated and transferrin saturation percentage is at baseline?

Here, both serum ferritin and transferrin saturation are significantly elevated.

Assuming the patient is asymptomatic and LFTs are normal, this profile is suggestive of iron overload. I assume Hb is normal?

Would do MRI to look for hepatic iron overload.

If you want to try to identify the basis for iron overload, you may have to do full HFE gene sequencing (and possibly other genes) to look for other variants/deletions beyond the "typical" polymorphisms commonly tested.