That’s Spirou #1392, btw.

This one really stood out to me because the background’s not remotely in the typical Peyo style. For me, it sort of hearkens back to various classic animations, in which simple, cartoony figures appear on top of gorgeous, hand-painted backgrounds. A couple of the early 60’s-era cartoons often used that effect, such as Hanna Barbera’s first couple Flintstone seasons, and Depatie-Freleng’s Paris-based The Inspector.

(right-click as needed)

https://static.tvtropes.org/pmwiki/pub/images/the_inspector_cartoon.jpg

https://pbs.twimg.com/media/EfgB9hwXsAEqWd2.jpg

A much earlier example were some of Fleischer’s Popeye cartoons. In the 1936 Sindbad one below, you can also spot a very interesting process at around the 8:40 mark where they have Popeye trekking through a cavern that’s not actually a painted flat source, but composed of at least two layers of rotating 3D background models(!)

https://www.youtube.com/watch?v=SvvJ_f_tBXI

Or a couple Moomin posters I found: (same post, different instances)
https://sopuli.xyz/post/35646382
https://piefed.social/c/moomin/p/1403905/zoom-in-on-the-moomins-3-posters