Place an oven rack in the bottom third in the oven, then preheat.
Brush the inside and tops of a standard 12-cup muffin tin or popover pan with melted butter or coat well with nonstick spray. It’s important to get the top as well, so the popovers do not break when removing.
Place eggs, milk, and Kosher salt in a blender, then process on medium-high speed until thoroughly mixed. Dump flour in all at once, then blend again. Add grated Gruyere cheese, melted butter, and cracked black pepper, then blend one final time until frothy.
Pour enough batter into each muffin mold to fill it half way. Add a couple cracks of black pepper on top of each, then carefully transfer to the preheated oven and bake. Reduce the oven temperature, then continue baking for another 10-15 minutes, or until the popovers have completely puffed up and are golden brown.
As soon as you pull the popovers out of the oven, poke the tops of each with a paring knife. This will release the steam and help them maintain their shape. Serve warm with butter or jam or alongside your favorite fall dinner!