Preheat your oven to 375°F (190°C).
Remove the stems from the portobello mushrooms and clean the caps with a damp cloth. Place them on a baking sheet lined with parchment paper, gill side up. Drizzle with olive oil and season with salt and pepper.
In a mixing bowl, mash the avocados and mix in lemon juice, garlic powder, onion powder, salt, and pepper until smooth.
Spoon the avocado mixture into each mushroom cap, creating a well in the center for the egg. Optionally, sprinkle shredded cheddar cheese on top.
Crack an egg into each mushroom cap on top of the avocado mixture. Bake in the preheated oven for 20-25 minutes until the egg whites are set.
Remove from the oven and let cool for a few minutes before serving. Garnish with fresh cilantro or parsley.