Laurent,, I find it really hard to believe you made that disc with non planar surfaces using sheet metal tools. I'm thinking it is a Revolve-Thin. The sheet metal form tools need a sheet metal body in order to work and also a flat surface to stop on. If you want to model this using non sheet metal features I would use the indent feature, fillet, and cut your hole in it. The indent feature also works on sheet metal non planar surfaces.. If your disc does not have any planar faces and is truly domed you would have to use the sheet metal swept flange to produce it and it would have to have an opening somewhere.
Edit:.. I wanted to add that since it looks like you used a spline for the profile you cannot use that in a swept flange. Therefore you will not be able to flatten it conventionally (with the sheet metal flatten).