In a large bowl, mix 1/2 the onions, 1/2 the garlic, and 1/2 the oil. Pour into a shallow, rimmed pan (12 by 15 in. to 15 by 17 in., and 1 to 1 1/2 in. deep). Repeat with remaining onions, garlic, and oil, and pour into another same-size pan.
Bake in a 400° oven until onions are limp and well browned, 1 to 1 1/2 hours, switching pan positions and stirring mixture from pan sides into center with a wide spatula every 15 minutes to avoid scorching or burning.
Use as directed in recipes. Or let cool to room temperature and freeze in 1-cup portions in plastic freezer bags (1-pt. size; see Cold Facts below).
Cold Facts. Packaging: freezer bags. If ingredients include liquid, set each plastic freezer bag upright in a bowl, then fill. Squeeze out all air and seal bag. For better protection of frozen foods during storage, seal each bag inside a second one. Lay bags flat in freezer until solid so they will stack.
Note: Nutritional analysis is per cup.