1. Begin by caramelizing the onions. Heat the olive oil in a small skillet over medium-low heat. Add the thinly sliced onions, a pinch of salt, and stir occasionally. Let them cook slowly for about 15-20 minutes until they are golden, sweet, and soft. Stir in the fresh thyme in the last few minutes, then remove from heat and set aside.
2. While the onions are caramelizing, prepare your bread and cheese. Spread a thin layer of softened butter on one side of each slice of bread. This will be the outside that crisps up in the pan.
3. On the unbuttered side of two slices, evenly distribute the shredded cheddar and fontina cheeses. Then, spread the caramelized onions on top of the cheese layers. If you’re using smoked paprika or cayenne, sprinkle a light pinch over the onions for that little extra warmth.
4. Top with the remaining bread slices, buttered side facing out. Press gently to help the sandwich come together.
5. Heat a cast iron skillet or heavy-bottomed pan over medium heat. Place the sandwiches in the pan and cook for about 3-4 minutes on one side, pressing gently with a spatula. You’re looking for that golden, crispy crust.
6. Flip the sandwiches carefully and cook the other side for another 3-4 minutes, until the cheese inside is melted and gooey and the bread is toasted to a perfect golden brown.
7. If your cheese isn’t melting as fast as the bread is toasting, lower the heat slightly and cover the pan with a lid to trap the heat and encourage melting without burning the bread.
8. Once done, remove from the pan, let the sandwich sit for a minute to cool slightly, then slice and serve.

I remember the first time I tried this method, watching Mama Lu’s patience with the onions and the slow melt of the cheese—it’s a small ceremony, but one that turns a simple sandwich into something you’ll want to savor. Don’t rush it; that’s part of the love.