In a large skillet over medium heat, melt butter. Add onion, carrot, and celery and sauté until soft, 7 to 8 minutes.
Add cauliflower and mushrooms and season with salt and pepper. Cook until tender, 8 to 10 minutes more.
Add parsley, rosemary, and sage and stir until combined, then pour over vegetable broth and cover with a lid. Cover until totally tender and liquid is absorbed, 15 minutes.