Step 1Pour milk into 8-quart or larger pot. Add sugar and stir with large wooden spoon until sugar dissolves and none is left at bottom.
Step 2Place pot over medium-high heat and add onion, if using, and cinnamon stick while mixture is still cold. Allow mixture to warm, stirring once or twice, until it begins to simmer. Bring mixture to boil without stirring (it should bubble up) 10 or 15 minutes. Do not allow to bubble over.
Step 3Reduce heat and cook mixture 1 to 1 1/2 hours, stirring frequently. Test for doneness by removing a teaspoon of liquid and allowing it to cool. It is done when it is cool and smooth and creamy, not runny. Allow it to cool. Remove onion and cinnamon stick.