If your mushrooms caps are very rounded, trim a very thin slice from the top of the caps to help them sit upright for baking. Toss the mushroom caps with the olive oil and 1/4 teaspoon salt in a large bowl.
For the filling:
Melt the butter in a large skillet over medium-high heat. Add the chopped mushroom stems and spinach and cook, stirring occasionally, until just soft and wilted, 6 to 8 minutes. Scrape into a large bowl, season with 3/4 teaspoon salt and a few grinds of pepper and set aside to cool.
Add the feta and basil to the cooled mushroom stem mixture. Stir in the breadcrumbs until well moistened.
Pack each mushroom with an equal amount of filling and transfer the filled mushrooms to a rimmed baking sheet. Bake until the mushrooms are tender and the filling is brown and crispy on top, about 15 minutes. Let cool 5 minutes before removing from the baking sheet.
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