This is under KDE. Locate boot.iso in your cooker tree. Right-click, choose "Open with" -> brasero. You will get the usual brasero burning popup requesting that you load a usable disk. Load a CD-RW (I keep a CD-RW to which I repeatedly burn the boot ISOs). The propmpt will change to recognize the volume, and all will be well EXCEPT that the "Burn" key is greyed out, with no hint of what to do to fix it. Most likely, this is related to the need to erase the CD-RW before use. No idea how old this is, I usually use K3B to burn to CD-RW; I haven't tried this with brasero for longer than I can remember.
Works differently now. 5Beta3 offers to blank the current disc
Status: NEW => RESOLVEDCC: (none) => nicResolution: (none) => FIXED