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1
Preheat the oven to 350 degrees F.
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2
Put a large sheet of parchment paper on a baking sheet.
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3
Arrange the lemon slices on the parchment, slightly overlapping, covering in an area just larger than the size of the tuna steaks.
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4
Season the tuna well on both sides with salt and pepper and arrange them directly on the lemon slices.
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5
Sprinkle the zest evenly over the tuna.
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6
Bring the 2 long sides of parchment together and fold down over the tuna; fold the ends of the parchment and tuck them under the fish like an envelope.
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7
Roast in the oven until cooked through, about 18 to 20 minutes; the tuna should flake easily with a fork and is no longer pink in the center.
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8
Open the parchment packet and let the tuna cool until it can be handled, then flake it with a fork.
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9
Transfer to a serving bowl along with any cooking juices it has released.
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10
Add the yogurt, grapes, and walnuts and stir to combine.
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11
Season with salt and pepper and freshly squeezed lemon juice, to taste.
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12
Serve immediately or refrigerate until ready to eat.
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13
Cook's Note: Serve this healthy salad on butter lettuce leaves, on a slice of toasted seedy whole wheat bread, with fresh cantaloupe slices, or eat straight out of the refrigerator with a fork!
