Cut the butter into 3 equal parts. Melt one part of the butter in a large and heavy nonstick skillet over medium heat. Once the butter is melted, add both slices of bread and cook, swirling around the pan occasionally, for about 2 minutes (bottom should be very lightly golden brown).
Transfer the bread slices onto a plate with the toasted side facing towards you. Add 2 cheese slices, meat, onions, and remaining 2 cheese slices (they should barely overlap on this bread). Close the sandwich so both of the toasted bread slices are facing inward.
Melt the 2nd piece of butter and reduce the heat to medium-low. Add the sandwich and cook, swirling around the pan occasionally until the sandwich is an even golden brown and the cheese is all melty (4-5 minutes). If you didn't add fried onions, add a pinch of salt to the sandwich. Serve immediately.
If desired, prepare the au jus according to package directions and serve alongside your simple grilled french dip sandwich!