Why "at" here instead of "by"? If you work at the restaurant, wouldn't that be "A drága étteremben dolgozom"?
I think you can say both: étteremnél and étteremben both can mean that you are employed at the restaurant. Étteremnél can also mean that you work something else close to the restaurant.
"I am working at" is not accepeted, although it has the same meaning as "I work" - at least in Hungarian.