Heat oven to 350 degrees. Meanwhile, melt butter in large Dutch oven over medium-high heat until foaming subsides, about 1 minute. Add onion and sauté until it turns soft and translucent about 4 minutes. Add garlic and sauté until fragrant, about 30 seconds. Add half and half, milk, thyme, bay leaves, salt, pepper, and potatoes and bring to simmer.