In a large skillet, heat the olive oil over medium heat. Add the chopped onion and minced garlic, sautéing until the onion is translucent, about 3-4 minutes.
Transfer the sautéed onion and garlic to the slow cooker. Add the chicken, carrots, potatoes, chicken broth, thyme, rosemary, bay leaf, salt, and pepper. Stir to combine.
Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is tender and the vegetables are cooked through.
If you prefer a thicker stew, mix the cornstarch and water in a small bowl to create a slurry. Stir it into the stew during the last 30 minutes of cooking.
Remove the bay leaf before serving. Taste and adjust seasoning if necessary. Serve hot.