1. Preheat oven to 375°.
2. 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.
3. 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.