Bring a large pot of salted water to a boil over medium heat and add the string beans. Blanch until bright green, about 3 minutes, then transfer to an ice bath. Warm the butter and the oil in a large skillet over medium heat. Add the onion and garlic, a pinch of salt and pepper, and cook until they are softened but have no color. Add the bell pepper to the skillet and continue to saute another 2 minutes. Strain the string beans and pat dry. Add the beans, tomatoes, turkey and lemon juice and stir until all ingredients are evenly incorporated and warm. Transfer to a serving bowl, top with toasted pine nuts and serve.