The lamb sits in a simple marinade seasoned with rosemary, garlic and sweet paprika, with a combination of lemon juice and olive oil as the medium to carry the flavors.
Trim lamb of silver skin, excess fat and sinew. Cut into 16 chunks about 2 inches square. Place in a bowl. Mix together 2 tablespoons oil, the lemon juice, paprika, 1 tablespoon rosemary, 2 cloves garlic (sliced) and salt and pepper. Pour over lamb and coat well. Cover, refrigerate and marinate 6 hours.
Heat remaining oil in a large skillet. Add mushrooms and cook over mediumhigh heat, turning once, just until they start to soften. Set aside.
Finely chop remaining rosemary and garlic together and place on a small plate. When mushrooms have cooled, lightly dip gill side of each cap into rosemary and garlic mixture. Arrange, herbed side up, on a platter.
About 30 minutes before serving, preheat a grill to very hot. Thread lamb on each of 4 skewers, using four pieces for each and sandwiching each piece of lamb between two mushroom caps, with herbed side against meat.
When grill is hot, grill lamb to desired degree of doneness, turning several times, about 15 minutes total for medium-rare.