Make the dough. In the bowl of a stand mixer fitted with a dough hook*, combine bread flour, yeast, salt and granulated sugar. Mix to combine.
Add sourdough discard. With the mixer on low speed, slowly pour in ¾ cup of warm water and the olive oil. Increase speed to medium until a shaggy dough forms and wraps around the dough hook. If needed, add more flour or water (1 Tablespoon at a time) to reach your desired consistency.
Transfer the dough to a lightly floured surface and knead for 2-3 minutes until a soft, smooth dough forms. The dough should not be sticky. Transfer the dough to a large oiled bowl and cover with plastic wrap. Let rise for 60-90 minutes, or until doubled in size.
Preheat oven to 450°F. Lightly dust two 12-cup muffin pans with semolina flour and set aside**.
Place the dough on a lightly floured surface and divide into 24 equal-sized pieces (you can eyeball this, or use a kitchen scale). Lightly shape each piece of dough into a round and use your fingers to press the dough into the bottom of each muffin cup. This doesn't need to be perfect. Cover the pans with plastic wrap or a clean kitchen towel and let rest for 10 minutes.
Top the pizzas. After resting, lightly press the dough into the bottom of the muffin cups again. Spread about 1-2 Tablespoons of tomato sauce onto each pizza, then top with shredded mozzarella and pepperoni (or other desired toppings).
Bake for 12-15 minutes, flipping and rotating the pans halfway through baking to ensure even cooking.
Let the pizzas cool slightly, then use a butter knife or spoon to run around the edge of the muffin cups and remove each pizza. Serve warm!
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Notes
1 serving = 3 miniature pizzas. Nutrition information is calculated including tomato sauce and cheese. To make this recipe with active sourdough starter, omit the yeast and replace the sourdough discard with active starter (200g). You will also need to increase the rise time.