Cut the beef into small cubes and grind it in 2-3 batches in your food processor, on pulse, until it has the consistency of ground beef.
Coat a large non-stick pan with cooking spray and place over medium-high heat. When pan is hot enough, add the mushrooms and cook them until they get all nice and golden brown. Transfer to a small plate and reserve.
Put the skillet back over the heat, add onion, celery and garlic and cook until fragrant and takes a slight coloration, about 3-4 minutes.
Add ground beef and continue cooking until the beef is no longer pink. Add miso, sambal oelek, mustard, tomato, zucchini and reserved mushrooms. Continue cooking for a few minutes until liquid has almost completely evaporated.
Transfer to an baking dish, and set aside.
Wipe your skillet clean and add diced eggplant. Cook over medium-low heat until soft and tender. Try not to let the aubergine take too much coloration. You want it to stay as white as possible.
Meanwhile, using a steam basket, steam the cauliflower florets and potatoes over salted water until fork tender, about 5-7 minutes.
Add cooked potatoes, cauliflower and aubergine to your food processor (make sure you clean your bowl first) and process until smooth. You could also use a hand mixer for this, especially if your food processor isn't big enough to accommodate all of the veggies.
In a separate mixing bowl, add buttermilk, yogurt, tahini paste, nutritional yeast, garlic, salt and pepper. Mix with a whisk until well combined and add to mashed potato mixture. Process again until well incorporated.
Arrange raw corn kernels over reserved meat mixture then carefully spread potato mixture on top.
Cook in the oven for about 15-20 minutes, until it starts to bubble and the top turns slightly golden brown. If you like a nice golden crust on top of your potatoes, you can brush a little bit of Extra-Virgin Olive Oil over the potatoes to help the browning process.