Prep ingredients. Slice loaf of bread in half lengthwise. If you'd like, you can scoop out a little piece of the center of the bread to help create a boat for toppings.
If you're adding mayo, add that to each side of bread. Add cheese slices.
Next, layer your meats.
Add tomato slices, lettuce and onion. If you'd like, drizzle olive oil and vinegar over the top, and then add a few cracks of salt and pepper.
Place the other half of your bread on top, gently pressing to seal. Slice and serve.
Notes
Fresh is Best - Especially when each ingredient will shine in sandwiches like this one. High quality deli meats will make a huge difference here, for the best results.
Make Ahead - Pack this sandwich to eat later, but don’t dress with mayo, oil and vinegar until just before serving.
Toast It - This sandwich is delicious with toasted bread, too. To toast, slice the bread as directed and add the cheese. Toast in 350 F oven for 4-5 minutes before adding meats and toppings.