Preheat the oven and prep the mushrooms - Preheat the oven to 425℉, convection bake setting. Brush off the mushrooms and remove the stems. Rub the mushroom caps with olive oil and place the mushroom caps, gill-side up, on a lined baking tray. Sprinkle with a bit of salt, and bake for 10 minutes. Pull the mushrooms out of the oven, and pour out the liquid that has pooled in the caps. You can discard the liquid or save it to add to vegetable stock.
Prep the vegetables and the bread crumb topping - If using frozen spinach, cook in a saucepan with a bit of water, until thawed. If using fresh spinach, chop, then blanch in boiling water. Drain and set aside. Mince the shallot and green onion. Mix 1/4 cup of the Parmesan with the bread crumbs.
Assemble and bake - Mix the spinach, shallot, scallions, yogurt, cream cheese, mayo, 1/4 cup Parmesan, dill, garlic powder, salt and pepper. Spoon a dollop of filling on each mushroom cap, then top with the Parmesan bread crumb mixture. Bake for 10 minutes or so, until the bread crumb mixture is toasted and the stuffed mushrooms are warmed through.