Preheat oven to 400 degrees Fahrenheit and grease a large baking sheet.
In a small bowl, whisk together stock, balsamic vinegar, and thyme.
In a large bowl, toss the sweet potato pieces with half of the spiced balsamic vinegar. Spread the coated sweet potatoes in a single layer on the prepared baking sheet. Sprinkle with salt and pepper. Roast in the preheated oven until almost tender and golden on the bottom, about 20 minutes.
Toss the sliced mushrooms in the remaining spiced balsamic vinegar. Flip the sweet potatoes and add mushrooms to the baking sheet with a slotted spoon. Arrange the vegetables in a single layer. Roast until the sweet potatoes are tender and mushrooms cooked, about 20 minutes.
Garnish with parsley and serve hot.