Prepare the Onions and Turkey: Heat a large skillet over medium-high heat. Add the olive oil and red onion slices to the skillet, cooking until softened and deeply browned, about 7 to 10 minutes. Crumble in the ground turkey with a pinch of salt, breaking up the meat as it cooks. Continue cooking until the turkey turns crisp and dark brown, approximately 8 minutes.
Mix the Sauce and Add Snap Peas: While the turkey is browning, whisk together the lime juice, fish sauce, red-pepper flakes, torn mint leaves, cilantro, and the light and white parts of the scallions. Pour the sauce into the skillet and add the sugar snap peas. Toss to combine. Cover the skillet and let the snap peas steam until tender, about 3 to 5 minutes.
Finish and Garnish: Taste the dish and adjust seasoning with additional fish sauce, lime juice, or salt as needed. Stir in the chopped cashews or peanuts, if using. Garnish with more torn mint leaves, chopped cilantro, and the dark green parts of the scallions. Serve immediately.