Preheat oven to 400 degrees F.Advertisement
If using fresh onions, bring a large pot of water to a boil. Add onions and cook 1 minute to loosen the skins. Drain. When cool enough to handle, trim both ends, leaving enough of the root end to keep the onions whole while roasting. Peel off the skins. Toss the prepared fresh onions (or frozen onions) with 1 tablespoon oil and 1/4 teaspoon salt in a large bowl. Spread in an even layer in a roasting pan large enough to accommodate all the onions in a single layer. (If you're using fresh onions, a 9-by-13-inch pan is large enough; if using frozen, you may need a larger pan.) Roast the onions, stirring occasionally, until soft and brown in spots, 45 minutes to 1 hour.
Remove the roasting pan from the oven and add broth, stirring and scraping up any brown bits. Return the pan to the oven and roast for 10 minutes more.
About 30 minutes after the onions start roasting, start the cream sauce. Melt butter with the remaining 2 tablespoons oil in a large saucepan over medium heat. Add flour and cook, whisking, until the mixture bubbles and is free of lumps, about 30 seconds. Whisk in milk, then add bay leaf, thyme, pepper and the remaining 3/4 teaspoon salt; bring to a gentle boil, whisking often. Reduce heat to the barest simmer and cook, whisking often, until the sauce has thickened to the consistency of thick gravy, about 5 minutes. Remove from the heat. Discard the bay leaf.
Stir the roasted onions and any broth from the pan into the cream sauce. Stir in lemon juice. Transfer to a serving dish and serve warm.
Make Ahead Tip: Reheat in the microwave on High, covered, stirring frequently, or on the stovetop over medium-low heat.
1 vegetable, 1 fat