Heat the oven to 200 C / 400 F with a rack in the middle. Wipe the mushrooms clean with a damp towel, twist out the stems, and chop the stems finely.
Toss the caps with 1 tbsp olive oil and the salt, then arrange them gill-side down on a 33 x 46 cm / 13 x 18-inch sheet pan, leaving 2 cm between them. Roast for 10 minutes, until the caps look wet and have released liquid.
While the caps roast, heat 1 tbsp oil in a 25 cm / 10-inch skillet over medium heat. Cook the chopped stems for 4 to 5 minutes, until their liquid has evaporated and they start to brown. Stir in the garlic for 30 seconds, until fragrant, then scrape into a bowl.
Add the peppers, feta, 25 g panko, parsley, oregano, lemon zest and black pepper to the stems. Mix until the filling holds together when pressed.
Flip the roasted caps hollow side up and tip out the liquid. Toss the remaining 15 g panko with 1 tbsp oil, then mound the filling into each cap, pressing lightly, and sprinkle with the oiled panko.
Roast for 15 minutes, until the feta edges are golden, the panko is browned and the filling sizzles at the edge. Rest on the pan for 5 minutes before serving.