• Heat the butter in a large cast-iron or nonstick skillet over medium-high heat. Add the onion and cook, stirring, until soft and just beginning to brown at the edges, 2 to 3 minutes. Add the corn and sugar (if using) and cook, stirring occasionally, until crisp-tender, about 3minutes. Scrape the corn into a bowl and let cool for 10 minutes (wipe the skillet clean and reserve). Stir in the 2 tablespoons mayonnaise, followed by the mozzarella, fontina and parsley, and season with salt and pepper.
• Brush a thin layer of mayonnaise on each slice of the sourdough bread, then turn the slices over. Divide the corn mixture among 4 slices of bread, then sandwich them with the other 4 slices. (Make sure the “mayonnaise sides” are facing outward.)
• Heat the reserved skillet over medium heat. Add 2 sandwiches to the skillet, cover, and cook, flipping once, until golden brown outside and the cheese is melted inside, 8 to 10 minutes. Transfer the sandwiches to a cutting board and repeat with the remaining sandwiches.
• As soon as they’re cooked, cut the sandwiches in half and eat, being careful not to burn your mouth on the molten cheese.

Variation:

• Elotes Grilled Cheese: Substitute queso Oaxaca for the mozzarella, substitute queso fresco for the fontina, substitute cilantro for the parsley, and add 1/2 teaspoon of Tajin to the corn mixture. Serve hot with a squeeze of fresh lime.