Wash and dry the lemons well. Finely zest them, taking only the yellow outer peel. Place the zest and 170g sugar in a bowl. Rub together with your fingertips for 2–3 minutes until very fragrant and lightly damp. Let sit for 20 minutes so the sugar absorbs the lemon oils. Rubbing rather than simply mixing the zest into the sugar is what actually ruptures the peel's tiny oil glands, mechanically releasing those aromatic oils directly into the sugar without needing any heat.

Place the rhubarb, 150g of the strawberries, 250ml water, and 60g sugar in a saucepan. Set over low to medium-low heat and cook gently for 8–10 minutes, stirring occasionally, just until the rhubarb softens and breaks down. Do not boil hard or reduce aggressively. Remove from the heat and stir in the remaining 100g strawberries. Let stand 10 minutes so the fresh strawberry flavor infuses without becoming cooked and jammy. Strain through a fine sieve, pressing gently to extract the juices. Do not force too much pulp through unless you want a slightly cloudier lemonade. Splitting the strawberries into two batches — some cooked alongside the rhubarb, the rest added only once the heat is off — is what lets this base carry two genuinely distinct qualities at once: real cooked depth from the rhubarb-and-first-batch simmer, and bright, fresh strawberry character from the portion that never touches direct heat.

Add the fresh lemon juice to the lemon sugar and stir until the sugar mostly dissolves. Let stand 2–3 minutes, then stir again. Strain through a fine sieve, pressing on the zest well, to create a fragrant lemon syrup. The lemon juice's own acidity helps dissolve the sugar while carrying the already-released lemon oils fully into the base, and pressing the spent zest firmly before discarding it captures as much of that flavourful syrup as possible.

In a large pitcher, combine the lemon syrup, the strawberry-rhubarb syrup, 900ml cold water, and the pinch of salt, if using. Stir well. Taste and adjust with a little more cold water if needed, depending on how concentrated your fruit base is. The final yield should be about 2 liters. This tasting checkpoint matters directly given how much rhubarb's own natural tartness can vary — building in that adjustment stage accounts for that variability rather than assuming a fixed recipe always lands at the same balance.

Refrigerate for 30 minutes, until thoroughly cold. That resting period lets the lemon and fruit components fully meld together, producing a more unified flavour than serving the drink immediately after mixing.

Fill glasses with ice. Garnish each with a thin fresh strawberry slice fanned at the rim, then pour in the lemonade and serve immediately. Serving right after chilling, rather than letting the pitcher sit out, keeps the drink at its coldest and most refreshing right when it's poured.