Shrek was eating onions to help himself warm up his layers. "Aye it's a good day to just enjoy nature around us."
"Easy for you to say." Pikachu commented upon sneezing so loudly it caused himself to set off a bunch of electrical bolts around the swamp. "Much as I love hanging around here I think I might be getting sick or something."
Shrek was quick to comfort Pikachu by offering him 1 of his onions since the ogre knew that the electric mouse Pokemon might need something to keep him in check; after all it's better out than in.