Boil the beets. In a medium pot, place the beets and add cold water to cover. Bring to a boil and cook for about 40-60 min. until easily pierced with a knife.
Drain, slip off skins, chop into small cubes and set aside.
Remove the stems from the mushrooms and chop the stems into small pieces.
In a medium bowl combine the chopped beets with the chopped mushroom stems. Sprinkle with salt, pepper, thyme and add 2 tbsp. of olive oil. Mix well.
Preheat the oven to 400°F. Line a baking sheet with parchment paper.
In a small bowl mix remaining 2 tbsp. of olive oil with minced garlic.
Place the mushroom caps on the baking sheet stem side up; brush on both sides with oil mixture.
Spoon the beet filling into each mushroom cap.
Bake for about 20-25 min. until the mushrooms are brown and soft.
Garnish with parsley. Serve hot or cold and enjoy!