In the bottom of a double boiler, bring 1 inch water to simmer. Combine eggs, lemon juice, and sugar in top of double boiler and cook until thickened, about 10 to 12 minutes. Remove from heat and cool completely.: The faint hiss of simmering water and the gentle warmth it provides are crucial here, because indirect heat prevents the eggs from curdling. You want a gentle steam, not a rolling boil, so watch for tiny bubbles forming around the edge of the pan. If the water boils vigorously the top bowl will get too hot and the custard can scramble, so reduce heat and skim off excess steam. A common mistake is using direct heat, which makes the sauce grainy rather than velvety.
Using a stand mixer or electric hand mixer, whip heavy cream until stiff peaks form. Fold in cooled custard sauce until combined.: As these ingredients warm together, you will notice the mixture transform from watery to silkier and slightly thicker, almost like a light pudding. Stir constantly with a heatproof spatula so the heat distributes evenly, and watch for the mixture to coat the back of a spoon, which is the visual cue for proper thickness. If you stop stirring, the mixture can stick and create uneven cooking, so keep a steady motion. Avoid letting the temperature spike, because that can give you lumps instead of a smooth custard.
In a large bowl, add grapes, pineapple, oranges, cherries, marshmallows, and banana. Pour cream sauce over and stir to combine. Refrigerate at least 8 hours or up to 24 hours before serving.: Once thickened, set the custard bowl over a cool surface or an ice bath to halt cooking, and stir occasionally to release steam and prevent a skin from forming. The aroma will shift from raw egg to a bright, lemon scented cream, and you will feel the weight of the mixture lighten as it cools. A common misstep is assembling while still warm, which wilts whipped cream and makes the salad runny, so patience here makes a marked difference.
Using a stand mixer or electric hand mixer, whip heavy cream until stiff peaks form: As you whip the chilled heavy cream , you will hear it change from a soft whisper to a thicker, swooshy sound, and visually it will climb the whisk into firm peaks. Stop when the peaks hold their shape, because overwhipping will convert cream into grainy butter. If you see small beads of liquid forming, you have gone too far, and you should start with fresh chilled cream for best texture.
Fold in cooled custard sauce until combined: Folding should be gentle and deliberate, using a broad spatula to lift through the cream and custard, preserving air while creating a homogenous sauce. The mixture will look glossy and billowy, with streaks of pale lemon merging into the cream. Vigorous stirring collapses the air and yields a denser result, so resist the urge to mix quickly. If the custard is still warm it will deflate the cream, so ensure it is fully cooled before folding.
In a large bowl, add grapes, pineapple, oranges, cherries, marshmallows, and banana: Combine the fruit so colors and textures layer nicely, and you will notice the bright reds and oranges pop against the pale cream when assembled. Toss gently so the fruit coat each piece without bruising softer items like the banana or mandarin segments. Pressing or over mixing can produce excess juice, which will thin the finished salad, so handle with a light touch.
Pour cream sauce over and stir to combine: When the luscious lemon cream lands on the fruit, you should hear a soft swoosh and see it glide through the bowl, enveloping each piece with a glossy finish. Stir carefully to ensure even distribution so every bite has a balance of fruit and sauce. Avoid vigorous mixing which can macerate the fruit and create an overly wet salad. The sensory cue here is uniform coating without visible pools of excess liquid.
Refrigerate at least 8 hours or up to 24 hours before serving: Chilling allows flavors to settle and textures to knit together, transforming separate components into a cohesive salad with softened marshmallows and melded citrus notes. You will notice aroma deepen and mouthfeel become silkier after rest. A trap is serving too soon, which results in a disjointed texture; patience yields the best result. Also, beyond 24 hours the fruit can break down and the salad becomes watery, so plan accordingly.