Generally speaking stainless steel is quite hard and will be quite a bit of work depending on the tools you have at your disposal.
I am going to assume this is a deep scratch.
Depending on how thick the plate is you may need to settle to scratch improvement, just in case there is a TON of material that needs to be removed to get down to the lowest level of the scratch.
What you will probably need is 60 to 100 grit sanding paper or sanding pads to start. You will need to sand along the grain of the (presumably) brushed finish, feathering the sanding outward from the scratch so that it doesn't build a depression - basically blending into the non damaged areas so it looks more natural
Once you have removed enough material to actually get down to a level you are happy with, the surface will be relatively dull. At this point you need to work up in grain on the papers, so if you started with 100, you would jump to 200, then 400 and so on, making sure to remove the previous paper's marks(scratches). Then you can finally finish off with 1000-1200 grit rubbing compound or rouge to polish the finish back to normal.
You are in for a little bit of work here, so it would be best to treat a smaller section at a time. I would also recommend doing test areas, to make sure the finish is what you want before you do it in a highly visible area.