1. Start by preheating your oven’s broiler or set a skillet over medium heat if you prefer a stovetop method. If you’re using the broiler, line a baking sheet with foil for easy cleanup.
2. In a small bowl, mix the mayonnaise and Dijon mustard together. Spread this mixture evenly on one side of each bread slice. This little step adds moisture and tang that balances the richness of the beef and cheese.
3. Layer the roast beef evenly on four slices of bread, then sprinkle shredded cheddar generously on top of the beef. Add thin slices of red onion if you’re using them, and season lightly with salt and pepper.
4. Top each sandwich with the remaining bread slices, mayo side down.
5. Spread butter on the outsides of each sandwich. Don’t skimp here—the butter helps create that golden, crispy crust that makes a melt melt-worthy.
6. If using a skillet, place sandwiches in the pan and cook for about 3-4 minutes on each side over medium heat, pressing gently with a spatula until the bread is golden and the cheese is melted. If using the broiler, place sandwiches on the baking sheet and broil for 2-3 minutes per side, keeping a close eye so they don’t burn.
7. Once the cheese is melted and bread is crispy, remove from heat. Let the sandwiches rest for a minute—this helps the cheese set slightly and avoids a lava-hot mouth.
8. Slice sandwiches in half and serve immediately. I like to cut mine diagonally for that classic deli feel!

One kitchen chaos story: I once tried to make these while simultaneously helping my kids with homework and ended up with one sandwich that was perfectly toasted and another that was just melted cheese on soggy bread. Lesson learned? Focus on one sandwich at a time, or enlist a helper (even if it’s a little assistant on a wobbly chair)!