burn (someone or something) to a cinder
1. To burn something, typically when cooking it, to the extent that it is completely charred and blackened. I forgot about the bread I'd put in the oven and burned it to a cinder.
2. To burn someone severely (typically referring to sunburn). Often used in passive constructions. If you lay on the beach all day without any sunscreen, you'll get burned to a cinder.
burned to a cinder
burned very badly. [Very often used figuratively.] I stayed out in the sun too long, and I am burned to a cinder. This toast is burnt to a cinder.