SGSX305TS/03 Thermador Cooktop Locked Burner Nut

I had someone come by to install some burner parts. He rounded one of the burner hold down nuts. Its brass I think. How do I get it out and where can I find a replacement?

Thanks!!

Added complete model number

via SGSX305TS/03 Thermador Cooktop Locked Burner Nut
by slackkeymike