Add the pepperoni to a soup pot over medium-high heat and cook, stirring occasionally, until the pepperoni render their fat and get crispy, 3 to 4 minutes. Remove the pepperoni with a slotted spoon and drain on paper towels. Set aside.
Lower the heat to medium; add the mushrooms, garlic, onions, oregano and 1 teaspoon salt. Cook, stirring, until the vegetables soften and the mushrooms release their liquid, 6 to 7 minutes. Add the broth, marinara sauce and Parmesan; cover the pot and bring to a boil. Reduce the heat to medium and cook, about 15 minutes.
Position an oven rack in the center of the oven and preheat the broiler to medium-high. Put the bread on a baking sheet. Brush the bread with the oil and sprinkle with 1/4 teaspoon salt. Broil the bread until crisp and brown, 1 to 2 minutes. Flip the bread and sprinkle with 1 cup mozzarella. Broil until the mozzarella melts and is brown and bubbly, about 2 minutes.
Ladle the soup into 8 serving bowls. Sprinkle each bowl with the remaining 1/2 cup mozzarella, then top each with a bread half, melted-cheese-side up, and some of the pepperoni. Sprinkle with some Parmesan and basil if desired.