Preheat oven to 350°.

Take a large jelly roll pan (at least 10 × 15 inches) and lightly grease bottom and sides.

Line the greased pan with parchment or waxed paper; grease the paper.

In a sauté pan, melt butter and add diced onion; cook until onion is limp; set aside.

Combine the eggs, mustard powder, salt, pepper, and onion mixture and whisk well.

Pour the mixture into the prepared pan and bake for 10-15 minutes, or until the omelet is set.

Remove omelet from oven and invert onto a piece of aluminum foil; gently peel off parchment paper.

Layer the omelet with the ham and Swiss cheese and then roll it up jelly-roll fashion and wrap tightly in aluminum foil.

Place the roll on a cookie sheet and return it to the oven for 10 minutes.

When baked, remove the foil, slice the omelet into thick rolls, and serve.