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1
Put oven rack in upper third of oven and preheat oven to 400F.
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2
Line a large shallow baking pan with foil, allowing 2 inches to hang over each end of pan.
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3
Wash leek pieces well in a bowl of cold water, then lift out and drain well.
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4
Whisk together honey, oil, salt, and pepper and add onions, shallots, and leeks, tossing to coat.
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5
Spread onion mixture in an even layer in baking pan.
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6
Scatter thyme on top, then cover with another large sheet of foil and crimp edges all around to seal, forming a package.
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7
Roast until very tender, about 1 hour.
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8
Carefully remove top sheet of foil (steam will emerge) and turn on broiler.
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9
Broil onion mixture 5 to 6 inches from heat until tops of onions are browned and caramelized, 2 to 5 minutes.
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10
(Check frequently, removing pieces as they brown.)
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11
Discard thyme.
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12
Whisk together oil, lemon juice, garlic, mustard, salt, and pepper in a large bowl.
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13
Trim endives and halve lengthwise.
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14
Cut in half crosswise, then cut lengthwise into 1/4-inch-wide strips.
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15
Just before serving, add endives to dressing along with spinach and chives and toss until lightly coated.
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16
Season with salt and pepper.
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17
3Divide salad among 6 plates and top with warm onion mixture.