Spread 4 slices of the bread with the mustard, then top with the cheese, hard-cooked egg slices, some thyme, and the remaining 4 slices of bread. Spread the top slice of each sandwich with a little of the butter. Heat a nonstick skillet over medium heat and add the remaining butter. When the butter is melted, place the sandwiches in the skillet, buttered-side up. Grill the sandwiches 5 minutes, turn, then cook 5 minutes more or until the cheese melts.