Steam the potatoes and cauliflower together until fork tender.
Save the water from steaming.
Put the steamed potatoes and cauliflower in a large bowl and using a potato masher, mash to the consistency you like. Add the vegan butter and steaming water as needed to get the desired moistness.
Add salt and pepper to taste.
Cover to keep warm.
Saute the shallots and mushrooms in a couple of tablespoons of the veggie broth (or you could use a teaspoon or two of avocado oil) and the thyme until the shallots are translucent and the mushrooms are brown.
Add the flour to combine with the onions and mushrooms, using a whisk.
Slowly add the rest of the vegetable broth, continuing to whish to remove any lumps. Note: I like the garfava bean flour because it is unrefined and very easy to work with.
Continue to add broth to get the sauce to your desired thickness. Season with salt and pepper to taste. Simmer for about 5 minutes to fully cook the flour and the sauce will continue to thicken. Add more veggie broth as needed.
Serve the mashed potatoes and cauliflower, topped with the mushroom gravy.