1 teaspoon kosher salt
1 cup all-purpose flour
1 1/4 cups milk
3 eggs, beaten
2 tablespoons chopped fresh thyme leaves
1/4 cup reserved pan fat from Sunday Roast Beef
*Cook's note: olive oil can replace cooking fat if none is available
Sift the salt and the flour into a large bowl. Add half the milk and all the eggs into the flour mixture and whisk until smooth. Whisk in remaining milk, and then the thyme. Cover; let batter sit at room temperature, for at least 30 minutes, ideally 45 minutes.
Heat oven to 450 degrees F. Divide the rendered beef fat or oil, 1/4 cup total, between a 12-cup nonstick muffin tin or 12-hole Yorkshire pudding tray. Heat in oven until the fat is almost smoking hot. Place a baking sheet on the bottom rack of the oven to catch any fat drippings - this will help prevent any oven fires!
Uncover batter and whisk one more time. Pour batter into each cup, about 3/4 full and immediately place back into the oven. The batter will sizzle when being poured into the hot fat. Bake in the oven for approximately 20 minutes or until risen and golden brown in color. Turn oven off and leave puddings in the oven for just 5 more minutes to help set. Remove puddings and serve while hot and puffed.