To prepare the sweetbreads for the grill, soak them in several changes of ice water for 2 or 3 hours.
Pick off as much of the membrane as possible; then simmer in acidulated water (1 quart water, 2 tablespoons lemon juice, 2 teaspoons salt) for 10 minutes.
Cool quickly under cold running water.
Arrange them on a plate, place another plate on top and weight the top plate to firm and flatten the sweetbreads and to squeeze out any excess water.
Refrigerate for 2 to 3 hours.
Cut the sweetbreads into 1 inch chunks.
Alternate on 4 skewers with the mushroom caps and tomatoes.
Serve with lemon wedges and Maitre d'Hotel Butter.