Heat a large skillet over medium-high. Add the oil and red onion slices to the skillet and cook until soft and deeply brown,7 to 10 minutes. Crumble in the ground turkey and a pinch of salt, breaking up the meat. Cook until crisp and dark brown, about 8 minutes.
While the turkey is cooking, whisk together the lime juice, fish sauce, red-pepper flakes and basil. Pour the sauce into the skillet and add the sugar snap peas. Toss until combined. Cover and let the snap peas steam until tender and cooked through, about 3 to 5 minutes.
Taste and add more fish sauce, salt and lime juice as needed to make everything bright and savory. Stir in chopped cashews or peanuts, if using. Top with more torn basil.