Preheat oven to 400ºF
Small-dice the onion, celery, jalapeño, garlic, sun dried tomatoes. Knife shred the Brussels sprouts (see photo and directions).
Wipe debris from the mushroom caps with a paper towel. Pop off the stems and discard or save for making vegetable broth. You can also remove the gills if desired (see FAQs below).
Cover a baking sheet with parchment paper, then arrange 6 mushrooms face down (small dice the 7th mushroom to be used in the stuffing). Bake for about 10 minutes or until the mushrooms soften and release their liquid. Drain the liquid, flip the mushrooms over and bake another 5 minutes.
While the mushrooms are baking, sauté the veggies as follows. Heat 1 Tablespoon of the vegan butter (or oil) in a large skillet over medium-high heat with a bit of the onion. When it begins to gently sizzle add the rest of the onion, celery, jalapeño, garlic, and diced mushroom. Sauté, stirring occasionally, for about 5 minutes or until the veggies are softened and the diced mushroom has released its liquid and begun to turn golden. Stir in the thyme, and black pepper.
Scrape everything from the skillet into a bowl.
Stir the ground flaxseed and water together in a small cup. Set aside for about 10 minutes to create a "flax egg."
In the meantime, heat the second Tablespoon of vegan butter (or oil) in the same skillet over medium-high heat with a bit of the shredded Brussels sprouts. When it begins to gently sizzle, add the rest of the Brussels sprouts, and the sun-dried tomatoes. Cook, stirring often, for about 5 minutes or until the shreds are softened and golden. It's okay if some begin to char.
Return the cooked veggies to the skillet, add the crumbs, and mix everything together.
Drizzle the flax egg over the mixture and stir well.
By now, the mushrooms should be par-baked and ready to stuff. Using your hands, press stuffing onto each mushroom.
Top with grated cheese and bake for 15 to 20 minutes or until the cheese is melted. As an optional step, remove the parchment paper and broil the stuffed portobellos for about 30 seconds to make the cheese more melted and golden – and to char some of the Brussels sprouts.