Melt butter in a small saucepan, add the sage, and cook until the butter smells nutty and the solids at the bottom turn golden brown, no more than 5 minutes. Set aside to cool slightly.
3 tablespoon salted butter, 5 sage leaves
Place the beef tallow in your popover pan well, then put the pan on the middle rack of your oven and preheat to 450°F so the metal and fat are scorching hot when the batter goes in.
2 tablespoon beef tallow
Add the room-temperature eggs and milk to a blender and blend on low until frothy, about 10–15 seconds.
4 large eggs, 1½ cups whole milk
Blend in the flour, salt, and pepper just until smooth. With the blender on low, slowly stream in the browned butter with sage, then stop blending and stir in the parmesan cheese by hand with a spatula so it doesn't end up becoming one with the batter. Let the batter rest for 15 minutes while the pan heats up.
1½ cups all purpose flour, 1½ teaspoon kosher salt, ½ teaspoon black pepper, ½ cup parmesan cheese
Carefully pull the hot pan from the oven, fill each well about ¾ full of batter, and bake at 450°F for 20 minutes, then 350°F for 10-15 minutes until tall, deeply golden, and crisp. DO NOT OPEN THE OVEN WHILE THEY BAKE.
Poke a small hole in the side of each popover to let steam escape so they stay tall with crisp shells. Serve right away while warm with gravy, jam, honey, or butter.