Preheat oven to 500°F. Remove and discard the ends of the potatoes. Cut each potato into 1-inch-thick disks.
In a large bowl, combine potatoes, melted butter, olive oil, thyme, salt, and pepper. Mix until potatoes are well coated.
Grease a metal 12- x 8-in roasting pan. Line potatoes so that they are not touching and in an even layer. Add any remaining butter mixture in the bowl to the roasting pan.
Roast in preheated oven for 20 minutes. Remove from oven, and using a spatula, flip each potato disk over. Return to the oven to roast for another 15 minutes.
Using a spatula, flip potatoes again. Pour vegetable stock into roasting pan and scatter the crushed garlic cloves throughout the pan. Return to the oven for 15 more minutes.
Allow to cool slightly before removing the potatoes to a serving platter. Spoon the pan sauce over the potatoes and garnish with fresh thyme leaves.