To make the dough -- In a mixing bowl, dissolve the yeast and sugar in the water. Let rest until foamy, about 5 minutes. Add the flour and salt; mix until blended. Knead the dough on a very lightly floured surface for 10 minutes or until smooth and elastic. Put it in a lightly oiled mixing bowl, cover loosely, and set in a warm place (70° to 80°F) until doubled in bulk, about 2 hours.
To make the pizza -- Put a baking stone on the upper middle rack of the oven and heat the oven to 475°F. In a small skillet over medium heat, brown the bacon. Drain on paper towels and set aside. On a heavily floured surface, flatten the dough ball. Roll the dough into a 12-inch circle, lifting and stretching from underneath with the back of your hands. (If the dough resists, let it rest for a few minutes and then resume rolling.) The outside edges should be about 1/4 inch thick, the center a bit thinner. Transfer the dough to a floured pizza paddle or the floured back of a baking sheet. Brush the dough with the olive oil and sprinkle the scallions evenly to within 1/2 inch of the edge. Sprinkle on the cheese and the reserved bacon. Transfer the pizza onto the baking stone in the oven with a quick jerk of the paddle. Check the pizza after 2 or 3 minutes and deflate any giant bubbles if need be.
To make the topping -- While the pizza bakes, whisk together the vinegar, mustard, and olive oil; season with salt and pepper. When the edges of the crust are lightly browned and the cheese is bubbling, 10 to 12 minutes, return the pizza to the paddle or transfer it to a cutting board. Toss the spinach with the vinaigrette and pile it on the pizza. Sprinkle with the chopped egg, slice with a chef's knife, and serve immediately.