Chop stems and leave caps whole In a large saucepan, combine 6 cups water with the lemon juice, add the mushroom caps, and bring the liquid to a boil.
Simmer the mushroom caps for 6 mintues, transfer tham to paper towels, and let them drain. (Blanching the mushroom caps will prevent them from wrinkling while they are baking.)
In a skillet, cook the bacon over moderate heat until it is crisp, transfer it to paper towels, and let it drain.
Pour off all but 1-1/2 tbsp of the fat from the skillet, in the remaining fat cook the onion, the garlic, and the mushroom stems over moderately low heat,stirring, until the vegetables are softened, and add the soy sauce, the sesame seeds, 1/2 cup of the bread crumbs, the bacon, crumbled, and pepper to taste.
Cook the mixture, stirring, for 1 minutes, or until it is slightly dry, divide it among the mushroom caps, mounding it, and transfer the stuffed mushrooms to a jelly-roll pan. The mushrooms may be prepared up to this point 1 day in advance and kept covered tightly with plastic wrap and chilled.
Sprinkle the remaining 1 tbsp bread crumbs over the mushrooms and bake the mushrooms in the middle of a preheated 325F oven for 7 minutes, or until the filling is heated through.
To make the filling crisper, the cooked mushrooms may be put under a preheated broiler about 4 inches from the heat for 1 minute. Garnish the mushrooms with the scallion.
Makes 24 stuffed mushrooms.