Preheat the oven to 425°F. Line a large baking sheet with parchment paper.
Cut the onion into 8 wedges and transfer to the baking sheet. Cut the potato into 1-inch pieces and transfer to the baking sheet as well. Then, add the cauliflower, garlic cloves, avocado oil, poultry seasoning, and lots of salt and pepper to the baking sheet. Toss the vegetables thoroughly to coat in the seasoning. Separating the wedges of onion to expose more edges.
Slide the vegetables into the oven and roast for 50 minutes (yes, 50 minutes), flipping and stirring them at the halfway point. You want deep caramelization and browning on all the edges.
Once the vegetables are sufficiently roasted and slightly cooled, transfer them to an upright, high speed blender. To the blender, add the miso, drained cashews, balsamic vinegar, liquid smoke, and vegetable stock. Slowly bring the speed of the blender up to high and blend until your gravy is completely smooth. If you’re finding it to be too thick, add more vegetable stock until you achieve a pourable, but still thick, consistency.
Check the gravy for seasoning at this point. If it needs more salt, vinegar, pepper, miso, or even more poultry seasoning to suit your taste, adjust accordingly.
Transfer the sheet pan gravy to a medium saucepan and bring it to a light boil. Serve the hot gravy with mashed potatoes and all of your other favourite holiday foods!