Step 1: Prepare the Filling
Heat the olive oil or butter in a large skillet over medium heat. Add the diced onion and sauté for 3-4 minutes until softened and translucent. Add the minced garlic and cook for another 1-2 minutes, stirring frequently to avoid burning.

Increase the heat to medium-high and add the ground beef to the skillet. Break up the meat with a spoon or spatula and cook until browned, about 6-8 minutes. Drain any excess fat from the skillet if necessary.

Reduce the heat to medium, add the chopped spinach, and stir until wilted, about 2 minutes. Season with salt, black pepper, paprika, and nutmeg (if using). Taste and adjust seasoning as needed. Remove the skillet from the heat and set aside.

Step 2: Assemble the Sandwich
Spread a thin layer of butter or mayonnaise on one side of each slice of bread. Place a slice of cheese on the unbuttered side of four slices of bread. Spoon a generous portion of the beef and spinach filling over the cheese, spreading it evenly. Top with the remaining slices of bread, buttered side facing out.

Step 3: Cook the Sandwiches
Heat a large skillet or griddle over medium heat. Place the sandwiches in the skillet and cook for 3-4 minutes on each side, pressing gently with a spatula to ensure even browning. The bread should be golden and crisp, and the cheese should be melted.

Step 4: Serve
Cut each sandwich in half and serve immediately with your choice of sides. Garnish with pickles or your favorite condiments, if desired.