Preheat your oven to 375°F and prepare a baking sheet with parchment paper or light oil coating. Clean mushrooms gently with a damp cloth, then carefully remove stems and use a small spoon to scrape out dark gills, creating room for filling.

Heat olive oil in a large skillet over medium heat, then sauté diced onion for 3 minutes until softened and fragrant. Add minced garlic and cook 1 minute more, stirring constantly to prevent burning before adding chopped spinach.

Cook spinach for 2-3 minutes until wilted and any liquid evaporates completely from the pan bottom. Remove from heat and let cool slightly, then stir in ricotta cheese, Parmesan, panko breadcrumbs, eggs, salt, pepper, and Italian seasoning until fully combined.

Fold chopped sun-dried tomatoes and fresh basil into the filling mixture gently, creating a cohesive but still chunky texture. Spoon filling generously into each prepared mushroom cap, rounding slightly so filling mounds attractively above the mushroom edges.

Arrange stuffed mushrooms on prepared baking sheet and sprinkle shredded mozzarella cheese evenly over top of each cap. Bake for 25-30 minutes until mushrooms release their liquid and tops turn golden brown and slightly crispy around edges.

Remove from oven and let cool for 5 minutes before serving, allowing filling to set slightly while mushrooms remain perfectly tender.