Cheesy Potato Soup With Peppered Scallions

Name: Cheesy Potato Soup With Peppered Scallions
Ingredients: For the soup:; 2 tablespoons unsalted butter; 1 yellow onion, finely chopped; 3 garlic cloves, minced; 1 tablespoon all-purpose flour; 2 pounds russet potatoes (about 3-4 large ones), peeled and cubed; 3 cups whole milk; 3 cups vegetable stock or water; 2 teaspoons kosher salt, plus more to taste; 1 sprig fresh rosemary; 3 sprigs fresh thyme; 1/2 cup sharp white cheddar cheese, shredded; 1/4 cup Parmesan cheese, shredded; 1/4 teaspoon smoked paprika; 1/2 teaspoon cayenne; Crème fraîche, sour cream, or Greek yogurt, for serving; For the peppered scallion topping:; 2 tablespoons extra-virgin olive oil; 20 scallions (about 2 handfuls); 8 tablespoons (1 stick) unsalted butter; 2 teaspoons lemon zest; 1 1/2 tablespoons freshly squeezed lemon juice; Kosher salt, to taste; 1 tablespoon freshly ground black pepper