In a food processor, combine flour, yeast and sugar. With the machine running, add the warm water and allow the dough to come together for around 20-30 seconds. Let the dough rest for 5 minutes, then add the olive oil, egg yolk and salt and run the machine for another 30 seconds. Place the dough in a large oiled bowl and cover. Allow the dough to double in size. Divide the dough in half. If you're only making one pizza, wrap half the dough in plastic and refrigerate. Place one or both circles of dough back in the bowl, cover, and allow to rest while preparing the onions and sauce.
Prepare the caramelized onions and sauce:
Heat 2 tablespoons olive oil in a large skillet on low heat. Add onions and a pinch of salt. Have a measuring cup full of water nearby. Cook the onions on low heat, stirring frequently, until the onions begin to brown. Turn up the heat to medium and continue caramelizing the onions. As a glaze develops on the bottom of the pan, add a few tablespoons of water and scrape up the brown bits, incorporating them into the onions. The heat will cause the water to evaporate. Repeat this process for approximately 25-30 minutes, keeping a close eye on the onions to make sure they don't burn. When the onions have turned a dark brown color and are both bitter and sweet in flavor, remove from the heat and set aside.
To prepare the sauce, puree tomatoes, olive oil, salt, pepper, dried oregano and garlic in a food processor.
Build the pizza:
Preheat the oven to 500 degrees F. Sprinkle some semolina flour on a large baking sheet. You can also use a pizza stone and peel if you prefer.
Stretch the pizza dough to approximately a 12 inch round and set it on the semolina dusted surface. Top with approximately 1/3 cup tomato sauce. Spread the tomato sauce evenly, leaving approximately an inch of space around the edge of the pizza to create a crust. Top with half the caramelized onions, a handful of arugula and then 1/3 cup mozzarella cheese.
Bake the pizza for 13-15 minutes. Top with crushed red peppers to taste. Serve immediately.