You can make the bechamel sauce three days ahead (let it cool completely before refrigerating). The onions can be cooked, peeled, and refrigerated overnight. Heat the onions and sauce together over low heat before serving.
I agree with the other reviewer, the measurement on the salt has to be incorrect. It was so salty it was not edible. With 1/2 tsp instead for 3/4 TBS, I would rate it 5 stars
Rating: 3 stars
Too much salt! The recipe definitely has a typo and must mean 3/4 teaspoon, not 3/4 tablespoon. The first batch of sauce was ruined with salt and I had to start over. The second batch was delicious though. I used about a 1/2 tsp the second time and also added a squirt of lemon juice in the end for some brightness, which I thought made a nice difference.