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Arrange a rack in the middle of the oven and heat the oven to 350°F.
Gently wipe 24 large mushrooms with a paper towel or clean kitchen towel to remove any dirt. Remove and discard the stems. Use a spoon to gently scrape out and discard the mushroom gills to make room for the stuffing.
Place the mushroom caps in a large bowl. Drizzle 2 tablespoons olive oil and season with 1/4 teaspoon of the kosher salt and 1/4 teaspoon of the black pepper. Gently toss to coat, making sure not to break the mushrooms. Arrange the mushroom caps stem-side up on a rimmed baking sheet (reserve the bowl).
Finely chop 1 small bunch chives to get about 1/4 cup. Finely grate 1 ounce Parmesan cheese (about 1/2 packed cup).
Halve 1 small lemon and squeeze the juice from one half into the now-empty bowl (no need to wipe it clean); reserve the second half. Add half of the chopped chives, half of the grated Parmesan cheese, 1 pound lump crabmeat, 1/4 cup mayonnaise, 1 teaspoon Dijon mustard, remaining 1/4 teaspoon kosher salt, and remaining 1/4 teaspoon black pepper to the bowl. Stir to combine.
Divide the filling between the mushroom caps. The filling will mound up, but press it down to fill the cavity. Sprinkle the tops of the stuffed mushrooms with the remaining grated Parmesan cheese.
Bake until the mushrooms are tender, the filling is hot, and tops are golden brown, about 25 minutes. Squeeze the juice from the reserved lemon half over the mushrooms, garnish with the remaining chopped chives, and serve.