Using a medium or large saucepan, melt butter over medium heat.
Crack the egg into the pan and break up the yolk but don't completely scramble. Sprinkle on black pepper to taste.
Let fry over medium heat until bottom is done and it's easily flippable. Flip egg and while you let other side cook, add onions, yellow mustard, and half slice of the provolone cheese onto only one side of the egg.
Flip empty side of egg over veggies and top with other slice provolone until melted. If your egg is already done cooking and top cheese has not yet melted, transfer to plate and microwave for 15-30 seconds.
Place completed omelette in the middle of the croissant like a sandwhich and add extra mustard before closing, if desired.