Preheat oven to 375F.
In a medium bowl, using a spoon, combine the ground turkey, cooked quinoa, feta cheese, chopped onion, salt, pepper and chopped herbs. Be gentle and try not to overwork the stuffing. It needs to be light and fluffy. Set aside.
Cut off the tops of bell peppers, about 1/2" - 3/4" thick, leaving stems attached to the tops. Carefully remove seeds and membranes from inside of the peppers. Sprinkle some olive oil on the inside of the pepper, then stuff with the ground turkey and quinoa, mix gently with a spoon. Do not press on the stuffing too hard otherwise the stuffing will end up being dense. If necessary, stuff an additional bell pepper.
Place the peppers in a baking dish, making sure they stand up right, sprinkle with more olive oil and freshly ground black pepper, and place the tops back on the peppers.
Bake at 375F for about 1 hour and 10 minutes (everyone's oven is different so start checking at 60 minutes), until the internal temperature of the turkey stuffing reaches 160F.
Remove from the oven, let the peppers rest for 5 minutes, then serve. When plating, garnish the peppers with freshly chopped herbs and add a spoon of sour cream on the side.