Heat the oven to 400 F (200 C) with a rack in the middle. Wipe the mushrooms clean with a damp paper towel rather than rinsing them, since wet caps soak up water and turn mushy. Gently twist out the stems, chop them finely, and set the caps aside.

Toss the empty caps with the 1 1/2 tablespoons of olive oil and the 1/4 teaspoon of salt. Arrange them gill-side down in a single layer on a parchment-lined baking sheet and roast for 12 to 15 minutes, until they have released their liquid and look shrunken and drier at the edges. Let them cool on the sheet while you make the filling.

Heat the remaining olive oil in a skillet over medium heat. Add the shallot and the chopped mushroom stems with a pinch of salt and cook, stirring now and then, until the moisture cooks off and the mixture turns golden, about 7 to 8 minutes.

Stir in the garlic and dried thyme and cook for 30 seconds, until fragrant. Add the cannellini beans, nutritional yeast, the remaining salt, and a few grinds of black pepper. Warm everything through, then mash about half the beans with a fork or potato masher so the filling holds together while some whole beans remain for texture. Cook for 2 minutes to drive off extra moisture, then take the pan off the heat.

Stir half of the panko, the lemon zest, and the parsley into the bean mixture. Taste and adjust the salt. Flip the cooled caps gill-side up and spoon the filling into each one, mounding it just above the rim and pressing it in gently so it stays put. Sprinkle the remaining panko over the tops and press lightly so it sticks.

Return the pan to the oven and bake until the filling is hot all the way through and the panko tops are golden, 15 to 18 minutes. Let them rest for 5 minutes, then scatter over a little extra parsley and serve warm.