Grab 2 bread slices and spread ½ teaspoon mustard on one side of each slice. Spread ½ tablespoon mayo on the other sides. Place 2 cheese slices on one slice, then layer your tomato slice on top. Press the second slice down gently so everything stays put while cooking.

Repeat step one with your remaining 2 bread slices and toppings to make your second sandwich. Set both aside for a second while you prep your skillet—this actually helps the cheese soften slightly before heat hits.

Place a cast-iron skillet or regular nonstick pan over medium-high heat for 2 minutes. You want it hot enough that butter sizzles immediately but not smoking—that's your sweet spot. Once hot, add ½ tablespoon of butter to the pan.

Once you hear that sizzle, place your first sandwich in the pan carefully. Don't move it around—let it sit for 3-4 minutes until the bottom turns golden brown and crispy. Press down gently with your spatula once during cooking to help cheese melt faster.

Flip your sandwich using a wide spatula (don't panic if a little cheese leaks out—that's normal and honestly delicious). Add the remaining ½ tablespoon butter to the pan and cook the second side for another 3-4 minutes until equally golden.

Transfer the cooked sandwich to a cutting board and repeat the cooking process with your second sandwich, adding the last tablespoon of butter to the pan. This ensures both get the same crispy treatment without cooling down while waiting.

Let both sandwiches cool for exactly 2 minutes before cutting diagonally. This pause lets the cheese set slightly so it doesn't ooze everywhere when you bite in. Serve immediately while still warm and gooey.