Butter one side of each slice of bread and place buttered side down on a plate.
Begin heating a grill pan or skillet over medium heat.
Brush each slice of bread with balsamic vinegar. Layer onion, spinach leaves, gorgonzola, dates, and walnuts on one slice of bread and place the other slice atop it, balsamic side in, buttered side up. Place in heated skillet and cook, using a spatula to press down on the top slice, until the bottom slice is browned and cheese is melting, 3-4 minutes. Flip and cook another two minutes, until browned and hot. Serve immediately.
Recipe by Baked In at https://www.baked-in.com/2013/04/29/date-walnut-gorgonzola-grilled-cheese/