Preheat the oven to 400°F (200°C). Line a baking sheet with baking paper.
In a large mixing bowl, combine the finely chopped portobello mushrooms, almond flour, grated Parmesan cheese, dried oregano, garlic powder, salt, and black pepper.
Add the beaten egg to the bowl and mix until the ingredients are well combined.
Spread the mixture onto the prepared baking sheet, pressing it down into a thin, even layer to form the pizza crust.
Bake for 15-20 minutes, until the crust is firm and lightly browned.
Remove the crust from the oven and spread the tomato sauce over the top.
Arrange the sliced mozzarella cheese over the sauce.
Return the pizza to the oven and bake for an additional 8-10 minutes until the cheese is melted and bubbly.
Remove from the oven and let cool for a few minutes before slicing.
Garnish with fresh basil leaves before serving.