Preheat the oven to 400°F. On the stovetop, heat the olive oil in a 10-inch cast-iron skillet over medium heat. Add the onion and red bell pepper and sauté until softened and fragrant, about 5–6 minutes. Stir in the kale and cook until just wilted, 3–4 minutes; season with salt and pepper. Add the cooked farro and sun-dried tomatoes, drizzle with a little more olive oil if needed, and adjust seasoning.