Set your oven to 400°F (200°C) and allow it to preheat thoroughly. Prepare a sturdy baking sheet by lining it with parchment paper. This step is crucial to prevent the mushrooms from sticking and to promote even heat distribution, ensuring they cook uniformly and develop a pleasing texture.

Coat the tops of the portobello mushroom caps evenly with 1 tablespoon of olive oil using a brush. Arrange them gill-side up on a baking sheet lined with parchment paper. Bake in a preheated oven at 375°F for 5 minutes, allowing them to release any excess moisture and become slightly tender.

While the mushrooms pre-bake, heat the remaining olive oil in a skillet over medium heat. Sauté the chopped onion until translucent, about 3-4 minutes, then add the minced garlic and cook until fragrant.

In a large mixing bowl, thoroughly combine the sautéed onion and garlic with breadcrumbs, shredded cheese, oregano, salt, and pepper. Stir the mixture until it is evenly blended, ensuring the breadcrumbs are fully coated with the cheese and spices. The mixture should appear uniform and slightly crumbly, with no visible clumps of cheese or breadcrumbs.

Once the mushrooms have finished baking, take them out of the oven. Carefully tilt each cap to pour out any excess liquid that has gathered inside. This ensures the filling remains firm and prevents it from becoming overly moist. Be gentle to avoid tearing the mushroom caps during this process.

Generously fill each portobello mushroom cap with the prepared breadcrumb mixture. Press down gently to compact the filling, ensuring it remains secure during baking. This helps the mixture adhere well, preventing it from spilling out. The filling should be level with the mushroom edges, creating a neat and even surface.

Place the stuffed mushrooms back into the preheated oven. Bake them for an additional 15 to 20 minutes. Watch for the tops to turn a rich golden brown and ensure the cheese is fully melted, bubbling gently, and slightly crisped at the edges.

While the mushrooms are baking, keep an ear out for a soft sizzle, which signals that the filling is cooking uniformly and the cheese is melting smoothly. This auditory cue ensures that the mushrooms are baking at the right temperature, allowing the flavors to meld together perfectly without burning.

Take the mushrooms out of the oven and let them rest for a few minutes to cool slightly. This helps the juices settle. Then, drizzle with lemon juice, adding a refreshing acidity that enhances the overall flavor profile of the dish.

Before serving, sprinkle the stuffed portobello mushrooms with freshly chopped parsley. This final touch not only enhances the visual appeal with vibrant green but also imparts a subtle freshness that complements the savory filling. Ensure the parsley is evenly distributed to achieve a balanced presentation and flavor profile.