Cook bacon in a large nonstick skillet over medium heat until crisp. Remove bacon from pan, and crumble. Discard drippings in pan. Add 1 cup onion and garlic to pan; cook for 5 minutes or until tender, stirring frequently. Stir in the potatoes; cover and cook for 15 minutes, stirring occasionally.
Combine crumbled bacon, 1/4 cup cheese, green onions, sour cream, salt, pepper, and soup in a large bowl. Add potato mixture; toss gently to combine. Spoon mixture into an 11 x 7-inch baking dish coated with cooking spray. Sprinkle with remaining 3/4 cup cheese. Cover with foil coated with cooking spray. Refrigerate 8 hours or overnight.
Preheat oven to 350°.
Remove casserole from refrigerator; let stand at room temperature 15 minutes. Bake casserole, covered, at 350º for 30 minutes. Uncover and bake an additional 30 minutes or until bubbly around edges and cheese begins to brown.