How to Make It
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 teaspoon each salt and 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 bread.