Preparation Divide cheese and walnuts equally among 8 bread squares. Top each with 2 watercress sprigs. Sprinkle with pepper and top with remaining bread squares, making 8 sandwiches total. Press together gently to adhere. (Can be made 4 hours ahead. Cover and chill.) Melt 3 tablespoons butter in large nonstick griddle or skillet over medium heat. Cook 4 sandwiches on griddle until golden brown and cheese melts, about 3 minutes per side. Transfer to cutting board. Repeat with remaining 3 tablespoons butter and 4 sandwiches. Cut sandwiches diagonally in half. Transfer to plates and serve.
Preparation Divide cheese and walnuts equally among 8 bread squares. Top each with 2 watercress sprigs. Sprinkle with pepper and top with remaining bread squares, making 8 sandwiches total. Press together gently to adhere. (Can be made 4 hours ahead. Cover and chill.) Melt 3 tablespoons butter in large nonstick griddle or skillet over medium heat. Cook 4 sandwiches on griddle until golden brown and cheese melts, about 3 minutes per side. Transfer to cutting board. Repeat with remaining 3 tablespoons butter and 4 sandwiches. Cut sandwiches diagonally in half. Transfer to plates and serve.