Combine first 3 ingredients in a small bowl. Spread about 1 1/2 teaspoons mustard mixture evenly over each bread slice. Divide cheese, pickles, and tomatoes evenly over each of 4 bread slices; top with remaining bread slices.
Heat a nonstick grill pan over medium-high heat. Coat pan with cooking spray. Add 1 sandwich to pan. Place a cast-iron or heavy skillet on top of sandwich; gently press to flatten. Cook 45 seconds on each side or until cheese melts and bread is toasted (leave cast-iron skillet on sandwich while it cooks). Repeat procedure with remaining sandwiches. Cut sandwiches in half diagonally.