Attach soufflé collars to 6 individual soufflé dishes, 7.5 cm (3 inches) in diameter, 4 cm (1½ inches) deep, so that the paper rises 4 cm (1½ inches) above the top. Slice 6 strawberries and divide them among the dishes. Blend the remaining strawberries to a coarse purée.
Pour the measured water into a small heatproof bowl and sprinkle over the gelatine. Set aside for 5 minutes, then stand the bowl in a pan half-filled with boiling water and simmer for 3–4 minutes, stirring occasionally, until the gelatine dissolves to a clear liquid.
Put the egg yolks, sugar and lavender petals in a large heatproof bowl and stand over a pan of simmering water so the base does not touch the water. Whisk with a hand-held electric whisk (or rotary hand or balloon whisk) for 10 minutes or until the eggs are very thick and pale and the whisk leaves a trail. Remove the bowl from the heat and continue whisking until cool.
Gradually fold in the dissolved gelatine in a thin, steady stream, then fold in the strawberry purée.
Softly whip the cream, then fold into the soufflé mixture with the food colouring, if using. Chill if the mixture is very soft.
Whisk the egg whites into stiff, moist-looking peaks. Fold a large spoonful into the soufflé mixture to loosen it, then gently fold in the remaining egg whites. Pour the mixture into the prepared soufflé dishes so that it stands above the rim. Chill for 4 hours or until set.
Peel back the paper collars and tuck a few trimmed lavender stems under the string to serve.