1. Separate the cloves from the garlic head, without peeling them. Rinse them under water and pat them dry gently. Peel and slice the onions.
2. Heat the olive oil in a skillet and sauté the onions until they become slightly translucent.
3. Meanwhile, wash the sweet potatoes and peel them with a peeler. Cut them into quarters lengthwise, then dice them. Place them in a container filled with water as you go. Drain them and pat them dry with a towel.
4. Add the diced sweet potatoes to the onions, along with the garlic cloves, and brown them on all sides by tossing them or gently flipping them with a spatula. Season with salt and pepper.
5. Sprinkle the thyme leaves over the vegetables (you can keep a sprig for garnish).
6. Place a lid on the skillet, reduce the heat, and let it cook for about fifteen minutes until the sweet potatoes are tender.