Bougie Grilled Ham & Cheese

Prepare the Caramelized Onions: Heat butter and olive oil in a skillet with high sides over low. Add onions, and cook, stirring often, until onions are caramelized, about 45 minutes. 

Prepare the Sandwiches: Divide Gruyère cheese among 4 of the bread slices; top evenly with sliced ham. Spread some of the Dijon mustard on each of the 4 remaining bread slices; top evenly with Caramelized Onions and goat cheese. Top with spinach, if using. Close sandwiches together, and spread 2 tablespoons of the butter evenly over the top bread slices. 

Heat a large cast-iron or nonstick skillet over medium-low. Place sandwiches, buttered side down, in hot skillet. Spread remaining butter on unbuttered bread slices. Turn sandwich occasionally, until bread is crisp and golden and cheese is melted, 5 to 8 minutes. Cut in half and serve.