Oregano and peas seem like a bad match. The musty, pungent herb and the fresh, bright legume would seem to fight. Maybe this hunch also stems from their divergent seasons: the spring of the peas, the summer of oregano.
But when concocting a chicken and peas dish for dinner recently, I had no fresh herbs in the house except slightly wilting oregano left over from some now-forgotten recipe. I wanted that extra element an aromatic herb would provide, so I said to heck with it, chopped a bunch of the leaves finely, and threw them in.
The combo was a revelation. Oregano does go with peas, and brings together the chicken, mushrooms and onions nicely.
I would caution that using fresh oregano is vital here, but maybe I would be wrong about that, too.