You can swap any fall veggie -- brussels sprouts, turnips, or cabbage -- into this warm salad. Just make sure you cut everything to about the same size for even roasting. Don't skip the celery leaves: they may seem delicate, but they pack a lot of flavor.
Heat oven to 425 degrees.
On two baking sheets, arrange carrots, onion, and squash; drizzle with 2 tablespoons oil, and season with salt and pepper.
Roast, turning once, until caramelized and tender, about 30 minutes.
Meanwhile, place lentils and shallot in a medium saucepan and cover with water by two inches.
Bring to a boil, then simmer, covered, until lentils are tender, about 20 minutes.
Drain; discard shallot. Season with salt and pepper.
Combine vinegar and mustard.
Pour remaining 3 tablespoons oil in a slow steady stream, whisking constantly
Toss lentils and celery with vinaigrette and season with salt and pepper.
Spoon over roasted vegetables. Garnish with celery leaves.
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