4 pounds baby white and red potatoes

1 Vidalia or other sweet onion, cut into wedges

16 to 20 cloves garlic, smashed

½ cup extra-virgin olive oil

3 tablespoons unsalted butter, melted

4 teaspoons dried rosemary

4 teaspoons dried thyme

1 teaspoon salt

1 teaspoon black pepper

Fresh thyme, for garnish