To make balsamic dressing, combine all ingredients in a screw-top jar. Microwave uncovered on high for 20 seconds or until honey is melted. Secure lid. Shake to combine.
Preheat oven to 200ºC. Place mushrooms, stem-side down, in a single layer (they might overlap slightly) in a large baking dish. Pour over half the balsamic dressing. Turn mushrooms and drizzle with remaining dressing. Season with salt and pepper.
Arrange spinach leaves over mushrooms. Sprinkle with feta and thyme leaves. Roast for 15 minutes or until mushrooms are just tender. Serve with mashed potatoes.