Sauté diced onion and minced garlic in hot oil in a 3-quart saucepan over medium-high heat until tender. Add broth and half-and-half; bring to a boil. Gradually stir in grits and salt. Cover, reduce heat, and simmer, stirring occasionally, 10 minutes or until thickened.
Add cheese, pepper, and nutmeg; stir until cheese melts. Pour grits into a lightly greased 11- x 7-inch baking dish; chill 8 hours.
Invert grits onto a flat surface; cut into 12 wedges. Spray top and bottom of each wedge with cooking spray; arrange wedges on a baking sheet.
Broil wedges 6 inches from heat 2 minutes on each side or until golden.