Place garlic and onion in a food processor; process until finely chopped.
Add 1/4 cup Parmigiano-Reggiano and next 4 ingredients (through pepper); process until almost smooth.
Add artichoke hearts; pulse until artichoke hearts are coarsely chopped. Spoon mixture into a 3-cup gratin dish coated with cooking spray; sprinkle evenly with remaining Parmigiano-Reggiano.
Bake at 400 for 15 minutes or until thoroughly heated and bubbly.