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1
Make sorbet:
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2
Bring sugar and water to a boil in a 1-quart saucepan, stirring until sugar is dissolved. Remove from heat and cool syrup.
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3
Puree raspberries and lemon juice in a food processor, then force through a fine-mesh sieve into a bowl, discarding seeds. Stir in sugar syrup and chill, covered, until cold, about 2 hours. Freeze sorbet in ice cream maker. Transfer to an airtight container and freeze until slightly firmer but still spreadable, about 30 minutes.
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4
Assemble terrine:
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5
Line loaf pan with 3 pieces of plastic wrap (1 lengthwise and 2 crosswise), leaving at least a 2-inch overhang on all sides, then freeze pan 10 minutes. While raspberry sorbet is in freezer, soften mango sorbet and mango ice cream in refrigerator. Mash mango sorbet in a bowl with a spoon until spreadable but not melted. Mash ice cream in another bowl in same manner.
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6
Evenly spread 1 3/4 cups mango sorbet in bottom of loaf pan (preferably with a small offset metal spatula), then top with 1 3/4 cups mango ice cream and half of raspberry sorbet. (If sorbets or ice cream become too soft for spreading, freeze 10 minutes between layering.) Repeat layering with 1 3/4 cups mango sorbet, 1 3/4 cups mango ice cream, and remaining raspberry sorbet (pan will be full). Cover top of terrine with plastic-wrap overhang, then wrap pan with more plastic wrap and freeze until terrine is hardened, at least 8 hours.
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7
To serve, unwrap pan, then open overhang and invert pan onto a chilled serving platter. Wet a kitchen towel with hot water, quickly wring it dry, and drape over pan 30 seconds. Lift off pan and peel off plastic wrap.
