Preheat an oven to 200 C / Gas 6. Toss the cubes of sweet potato with 2 tablespoons of olive oil in a bowl. Season to taste with salt and pepper, and arrange evenly onto a baking tray.
Bake in the preheated oven until the sweet potatoes are tender, 20 to 25 minutes. Cool to room temperature.
Meanwhile, heat the remaining 1 tablespoon of olive oil in a large pan over medium heat. Cook and stir the onion and garlic until the onion has caramelised to a golden brown, about 15 minutes. Stir in the kale, cooking until wilted and tender. Transfer the kale mixture to a bowl and set aside.
Once all the ingredients have cooled, combine the roasted sweet potatoes, kale, red wine vinegar and fresh thyme in a bowl. Season to taste with salt and pepper, and gently stir to combine.