Cut potatoes into chunks and cauliflower into small florets. Steam for 15 minutes.
While the potatoes are cooking, heat oil in a large skillet.
Add diced onion and garlic to the skillet and sauté over medium heat for 5 minutes.
Finally, add sliced mushrooms and cook for another 7-10 minutes. When mushrooms are almost ready, season with salt and pepper.
Mash potatoes and cauliflower with a potato masher adding as much reserved cooking liquid as needed.
Season potatoes with salt and pepper. Serve mushrooms on top of mashed potatoes sprinkled with fresh dill.