Toss the onion and vinegar in a small bowl.
Let sit until the onion is tender and bright pink, 3 to 5 minutes.
Meanwhile, heat the butter in a medium skillet over medium heat.
Add the peas and ¼ tsp. salt and ¼ tsp. pepper and cook until heated through, 5 to 7 minutes.
Mash until the peas hold together.
Divide the peas, onion, pesto, and turkey among the rolls, serve open faced.