Simmer 1 1/4 pounds diced Yukon gold potatoes in salted water until tender, 12 minutes. Drain and pat dry. Heat 3 tablespoons vegetable oil in a large nonstick skillet. Add the potatoes and cook, stirring, until crisp, 8 minutes. Add 3 ounces diced pepperoni and 1 minced garlic clove; cook 1 minute. Stir in 3 chopped scallions and season with salt and pepper.