Preheat the oven to 375F and press the tofu between two layers of towels or paper towels to remove any extra moisture.
Then, prep the veggies by dicing the leek, mincing the garlic, finely chopping the rosemary, and removing the stems from the portobello mushroom caps. If desired, you can also scrape out the gills from the inside of the portobello mushrooms. It is also a good idea to make the flax egg at this time and set it aside for later.
Line a baking tray with parchment paper and place the mushroom caps onto the tray, stem side down. Lightly brush or spray the mushroom tops with oil and then sprinkle with salt & pepper to season.
Add the tray of mushrooms to the oven to bake for about 10 minutes until they begin to soften. Then, remove from the oven, flip over, and set aside until the filling is done.
While the mushrooms are baking, make the tofu leek filling. Add the vegan butter to a pan and melt over medium heat.
Once the butter has melted and is bubbling, add in the chopped leeks and minced garlic. Saute for about 2-3 minutes until fragrant.
Crumble the block of tofu into the pan and saute with the vegetables for around 8- 10 minutes or until the tofu begins to crisp up.
Next, add the breadcrumbs, thyme, rosemary, dried parsley, salt, & pepper to the pan. Saute with the tofu and vegetables for another 1-2 minutes. Then, remove the pan from the heat and stir in the flax egg.
Add the tofu leek filling to the stuffed mushrooms until they are completely full, then add back to the oven to bake for another 12-15 minutes.
Remove the mushrooms from the oven and serve immediately with additional fresh herbs or even vegan parmesan sprinkled on top. Enjoy!