1
Heat oil in a 10-inch stainless steel skillet over medium heat until shimmering. Add onion and cook, stirring frequently, until softened but not browned, about 5 minutes. Add spinach, cover skillet, and reduce heat to medium-low. Cook, without stirring, until spinach leaves release their water, about 2 minutes. Uncover and continue cooking, stirring occasionally, until most of the water has evaporated, about 10 minutes longer. Add garlic and cayenne pepper and cook, stirring, until garlic's aroma has mellowed, about 2 minutes. Add artichoke hearts and cook until they begin to fall apart, about 5 minutes more. Remove from heat and cool until no longer steaming, then stir in mayo. Season to taste with salt and pepper.
2
Spread about 1/2 tablespoon (7g) butter onto one side of each slice of bread, then transfer slices, butter side down, to an electric griddle preheated to 350u00b0F (177u00b0C) or a large cast iron skillet preheated over medium heat. (You may need to work in batches.) Divide cheese evenly over bread slices, then top half of them with spinach and artichoke mixture. After about 5 minutes, when cheese has fully melted and bread is golden, close sandwiches with a flexible metal spatula, transfer to a cutting board, and cut diagonally with a chef's knife. Serve immediately.
