This is the glass lid for Scanpan's Classic roaster 39 x 27 cm (item no. 35321200): tempered glass and stainless steel, 35.5 x 26.5 cm. It holds in steam and moisture, so the roast stays juicy and the gravy doesn't boil off and burn into the corners.
The glass lets you check progress without lifting the lid. Less dramatic than peeking every five minutes, but the roast will thank you.