In a small bowl, toss the pear slices with the lemon juice. Heat a large skillet or griddle over low heat.
Meanwhile, toss the turkey and thyme in a medium bowl. Spread each bread slice with mustard. Arrange half of the Brie on four slices of the bread. Layer the pears over the Brie. Mound the turkey mixture on top of the pears, layer on the remaining Brie, and top with the remaining bread slices mustard side down.
Lightly spread the tops of the sandwiches with half of the butter and set them, buttered side down, in the heated skillet (if necessary, cook the sandwiches in two batches). Set a large heavy skillet right on top of the sandwiches and put 2 lb. of weights (canned goods work well) in the empty skillet. Cook the sandwiches until golden brown on one side, about 4 minutes.
Remove the weights, butter the sandwich tops, and turn the sandwiches over. Replace the skillet and weights and continue to cook until the second side is golden brown and the cheese is oozy, about 4 minutes longer. Cut the sandwiches in half and serve.