Step 1

Arrange a rack in the top third of the oven and heat the oven to 400ºF. Line a rimmed baking sheet with parchment paper.

Step 2

Remove the stems from 4 large or 6 medium portobello mushrooms. Use a spoon to gently scrape out and discard the mushroom gills to make room for the stuffing. Brush the mushrooms all over with 2 tablespoons of the olive oil, then season all over with 1/2 teaspoon of the kosher salt and 1/4 teaspoon of the black pepper.

Step 3

Place the mushrooms on the baking sheet stem side down. Roast for 10 minutes for medium mushrooms, 12 minutes for large. Meanwhile, make the filling.

Step 4

Place 1 large egg in a medium bowl and beat lightly with a fork. Coarsely chop 2 garlic cloves and place in a regular or mini food processor fitted with the blade attachment. Add 1 cup raw California walnuts and pulse until chopped into pieces no larger than a peppercorn, 15 to 25 (1-second) pulses. (Alternatively, finely chop both by hand.) Add to the bowl with the egg.

Step 5

Prepare the following, adding each ingredient to the bowl as you complete it: Grate 4 ounces mozzarella cheese on the large holes of a box grater (about 1 cup). Grate 1/2 ounce Parmesan cheese on the smaller holes of a box grater (about 3 tablespoons), or use 2 tablespoons store-bought grated. Finely chop 1 small bunch fresh chives until you have 1/4 cup; reserve 1 tablespoon for garnish and add the remaining to the bowl.

Step 6

Add the remaining 1 tablespoon olive oil, remaining 1/4 teaspoon kosher salt, 1/2 cup panko breadcrumbs, and 1/4 teaspoon red pepper flakes. Stir until evenly combined and moistened.

Step 7

When the mushrooms are ready, remove the baking sheet from the oven. Flip the mushrooms and use paper towels to blot up the excess moisture from the mushrooms and baking sheet. Stuff the mushrooms with the filling.

Step 8

Return the baking sheet to the oven and roast until the mushrooms are tender and the filling is browned in spots, 10 to 15 minutes. Garnish with the reserved chives.