Preheat the oven to 400°F. Place a wire rack inside a foil-lined rimmed baking sheet; set aside.

Remove the stems from the mushrooms and wipe the caps clean using a paper towel; reserve the stems for another use, if desired. In a large bowl, toss the mushroom caps with the olive oil and 1/2 teaspoon of the salt. Arrange the mushrooms, cap side up, on the prepared baking sheet. Roast until the mushrooms are mostly tender and have released liquid onto the baking sheet, about 20 minutes.

Meanwhile, in the same medium bowl used to toss the mushrooms, stir together the cream cheese, cheddar, parmesan, pimientos, parsley, garlic, and the remaining 1/2 teaspoon of salt until well combined. Gently fold in the crabmeat.

In a small bowl, stir together the panko and butter. Set aside.

Stuff each mushroom cap with 2 to 3 teaspoons of the crab mixture and sprinkle with about 1/2 teaspoon of the panko. Return the stuffed mushrooms, cap side down, to the wire rack-lined baking sheet. Bake until the panko is golden brown and the crab filling is hot and melty, 15 to 20 minutes. Garnish with chopped parsley and serve immediately.