Preheat oven to 350°F. In the bowl of a food processor fitted with the metal blade, combine the mushrooms with the lemon juice and the onion. (This might have to be done in 2 batches if the food processor is small. Simply add half the lemon juice and half the onion with each batch.) Process with quick pulses several times, scrape down sides, and repeat until mushrooms are finely chopped and onion is incorporated.

In a heavy-bottomed, nonreactive saucepan, combine the mushroom mixture with the red pepper and celery. Cook over moderately high heat until mushrooms start to exude moisture. (Mushrooms will give off much liquid. This evaporates quickly and will result in a smooth paste that will help bind the meat loaf) Stir constantly until mixture is very thick and all liquid has cooked off, about 10 minutes. Empty into a bowl and allow to cool to room temperature before proceeding.

This recipe can be done in advance to this point. After coming to room temperature, cover tightly and refrigerate until ready to use.

In a bowl, combine the cooked mushroom mixture, turkey, breadcrumbs, parsley, and egg. Work quickly, stirring and/or mixing just enough to blend. Do not overwork or meat loaf will be tough. Season with salt and pepper. (To avoid eating raw turkey, cook a spoonful or two in vegetable oil before adjusting the seasoning.)

Lightly oil a 1-quart loaf pan with olive oil. Gently pack in the turkey and mushroom mixture. Cover loosely with an oiled piece of aluminum foil. Bake until there are no traces of pink and juices run clear, 45 minutes to 1 hour. Let rest in pan for 10 minutes. There will be an excess amount of liquid given off by the vegetables as the loaf cooks. Use a spatula or a small plate to hold the meatloaf in place as you tilt the pan to pour off excess liquid. Carefully unmold onto a serving platter. Pour off excess liquid again and spoon over a small amount of warmed Basic Tomato Sauce . Serve the remaining sauce on the side. Garnish with additional chopped parsley if desired.

Prehear oven to 350°F.