Cook the bacon strips in a skillet over medium-high heat until crispy, about 5-7 minutes, turning occasionally for even cooking. Transfer to a paper towel-lined plate to drain and cool slightly, then break or chop into bite-sized pieces. Set aside while you prepare the other components.

While the bacon cooks, spread 1 tablespoon of mayonnaise on one bread slice and 1 tablespoon of blueberry jam on the other slice. On the mayonnaise side, layer half the shredded cheddar cheese, then add the cooked bacon from Step 1, the sliced jalapeño, and the remaining cheese on top. Close the sandwich gently and sprinkle the outside with fresh thyme leaves and a pinch of flaky sea salt for extra flavor.

Heat a skillet or griddle over medium heat. While it preheats, spread about 0.75 tablespoon of butter on the outside of the top slice of your assembled sandwich and 0.75 tablespoon on the outside of the bottom slice. Once the pan is hot (a drop of water should sizzle), place the sandwich butter-side down in the pan and cook for 2-3 minutes until golden brown and crispy. Flip carefully and cook the other side for another 2-3 minutes until equally golden. I like to press down gently on the sandwich with a spatula while it cooks to help the cheese melt evenly and create a flatter, more compact sandwich.

Once both sides are golden brown, if the cheese hasn't fully melted yet, reduce heat to low and continue flipping the sandwich every 30 seconds for 1-2 minutes until the cheese is completely melted and gooey inside. Transfer to a plate and let it cool for 1-2 minutes before cutting and serving—this allows the cheese to set slightly so it won't ooze out when you bite into it.