Preheat oven to 425°.
Heat a 10-in. cast-iron or other ovenproof skillet over medium-high heat.
Add pepperoni and onion; cook and stir until pepperoni is crisp and onion is tender, 6-8 minutes.
Remove and keep warm.
In a large bowl, beat egg, milk, yogurt and oil until well blended.
In another bowl, whisk flour, baking powder, garlic powder and salt; gradually beat into egg mixture.
Stir in ½ cup cheese and pepperoni mixture (batter will be thick).
Transfer to same skillet; sprinkle with remaining ¼ cup cheese.
Bake until bread is golden brown and a toothpick inserted in center comes out clean, 20-25 minutes.
Cool 10 minutes in skillet on a wire rack.
Serve warm. If desired, serve with marinara sauce and top with fresh basil leaves.
