Preheat oven to 425°. Sift together first 4 ingredients into a bowl. Whisk together milk and next 2 ingredients in another bowl; add to flour mixture, stirring just until moistened. Whisk in grits. Spoon into greased 12-cup muffin pans (1 Tbsp. per cup).
Bake, in batches, at 425° for 12 to 15 minutes or until crostini is golden. Immediately remove from pans to wire racks; cool 30 minutes. Reduce oven to 375°.
Meanwhile, prepare Caramelized Apples and Onions.
Place crostini on a baking sheet. Top each with 1 tsp. Caramelized Apples and Onions; sprinkle with cheese. Bake at 375° for 8 minutes or until cheese is melted. Sprinkle with thyme; serve immediately.