Lightly coat a 1 1/2–quart soufflé dish or round casserole with cooking spray. In a medium bowl combine the next 8 ingredients (through salt). Stir in 1/2 cup of the cheese. Transfer egg mixture to prepared dish. Top with remaining 1/4 cup cheese.
Place dish in a 5- to 6-quart slow cooker. Cover and cook on high 2 to 2 1/2 hours or until frittata is set. (Frittata will puff during cooking and settle when cooling.) Let stand 10 minutes. Sprinkle with chives.