1. Dough: In bowl, stir together 1/4 cup flour, warm water, yeast and sugar; let stand for 5 to 10 minutes or until mixture starts to look bubbly and frothy. Using wooden spoon, stir in olive oil, remaining flour and salt to make loose dough. Transfer to lightly floured surface; knead for 10 to 15 minutes, adding flour as needed, to make soft, elastic, and slightly sticky dough.
2. Form dough into ball; transfer to lightly greased bowl. Brush top of dough lightly with oil; cover with wet kitchen towel. Let stand in warm spot for about 2 hours or until doubled in size. Punch down dough; transfer to airtight container or resealable plastic bag. Refrigerate overnight; let stand at room temperature for 2 hours.
3. Toppings: Place 12-inch cast iron skillet in oven and preheat to 500ºF. Meanwhile, heat 1 tbsp olive oil in large skillet set over medium-high heat; cook onion, red pepper, fennel, salt and pepper, stirring frequently, for 6 to 8 minutes or until vegetables are softened. Transfer to plate; wipe skillet clean.
4. Heat 1 tbsp olive oil in skillet set over medium heat; cook sausage, breaking up into smaller pieces with wooden spoon. Cook for 3 to 5 minutes, stirring occasionally, until browned and cooked through. Remove from heat and set aside.
5. On lightly floured surface, roll out dough into 14-inch round. Add remaining oil to preheated cast iron skillet. Carefully transfer dough to heated skillet, pressing evenly up side of skillet.
6. Spread pizza sauce over dough. Sprinkle with half of the mozzarella; top with sautéed onion mixture and cooked sausage. Sprinkle with remaining mozzarella and chili flakes.
7. Return skillet to oven; bake on bottom rack for 20 to 25 minutes or until crust is golden and crisp and cheese is melted. Let cool slightly.
Tip: For a zesty kick, try using hot Italian sausage – or for a sweeter note, use a honey-garlic or sweet Italian sausage.