To begin, combine 1 cup of the warm water, the sugar, and yeast in a small bowl. Stir together and let sit for 10 minutes until the the yeast is bubbly.
Meanwhile in a mixing bowl, combine the bread flour, dry mustard, and sea salt. Stir those together until thoroughly mixed.
Make a deep well in the center with your hand or the back of a spoon. Pour the bubbly yeast mixture into the well.
With your fingers, gently swirl the mixture around, pulling in enough of the surrounding flour to make the liquid the consistency of pancake batter.
Grab some of the flour from the outside and sprinkle it over the top of the liquid mixture to prevent a skin from forming. Cover the bowl with a damp towel and let it sit for 20 minutes.
Pour in the second cup of warm water. Then, with your hands, begin pulling the flour into the center, stirring until it comes together into a messy dough. Scoop the dough out of the center and place it on a lightly floured surface. Use the extra flour mixture that it still in your bowl. Knead the dough for 8-10 minutes, adding additional flour mixture as needed. The dough will start out as a shaggy mess - and after you have kneaded it, you will have a soft, smooth dough. Discard any remaining flour in the bowl. Wash, dry, and oil the bowl. Place your dough ball in the bowl and turn it over to lightly coat the dough with a thin layer of oil. Cover it with the damp towel again and let it rise at room temperature for 1 1/2 – 2 hours.
When the dough is nearly finished rising, grab the onion. Peel off the skin and cut 5 or 6 thing slices from one of the ends. Set the slices aside for now and dice the remaining onion.
In a skillet, sauté the diced onion in the extra virgin olive oil over medium low heat until the onion is tender and golden – about 5 minutes. When the onion is done sautéing, spoon it out onto a plate and spread it out so that it can cool.
While the onion is sautéing, get your Swiss cheese ready. Cut your block into two portions – about 2/3 and 1/3 of the block respectively. This does not have to be exact, just “eyeball it.”
Take the larger portion and shred it. Cut the smaller portion into 1/2 inch cubes.
Turn the dough out onto a lightly floured surface. Roll or pat it out into a large rectangle – about 12-18 inches.
Reserving 1/2 cup of the shredded Swiss, spread the remaining shredded and cubed cheese all over the dough. Sprinkle the sautéed onion all over the dough as well.
Begin rolling the dough up snugly from one end. When it is completely rolled up, pinch the edges together. Place the roll seam-side-down on a baking sheet or stone.
Place the roll seam-side-down on a baking sheet or stone. Tuck the ends underneath. Cover this with the damp towel again and let it rise for an hour.
Heat your oven to 400 degrees.
Brush the top of the loaf with milk.
Pile the reserved cheese down the center of the loaf. Bake this for 20 minutes.
While it is baking, bring the onion slices to a boil in a small skillet with 1/2 inch of water. Let it bubble for a minute then removed it from the heat and drain out the water.
After the bread has baked the 20 minutes, pull it out and carefully arrange the onion slices over the top of the bread. Put it back in the oven and bake for 20-25 more minutes, or until it sounds hollow when tapped and is golden brown on top. Slide the loaf over to a rack and cool.
Enjoy this wonderful bread with a bowl of soup…or as a scrumptious snack.