Based on the old English Yorkshire pudding, popovers are a quick bread classically paired with beef dishes. Make the popover batter up to two hours ahead and refrigerate until 15 minutes before you plan to bake them. Popover cups are tall and narrow so the batter "pops over" the top as it bakes; you can find a popover pan at any kitchenware shop. Muffin cups will also do (with five minutes less time in the oven), though they won't puff quite as dramatically.
Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, thyme, and salt, stirring with a whisk. Combine milk and eggs in a medium bowl, stirring with a whisk until blended; let stand 30 minutes. Gradually add flour mixture to milk mixture, stirring well with a whisk. Stir in melted butter.
Coat 8 popover cups with cooking spray; sprinkle cheese evenly among cups. Place cups in oven at 375° for 5 minutes. Divide batter evenly among prepared cups. Bake at 375° for 40 minutes or until golden. Serve immediately.