Add the chopped rhubarb and strawberries to a medium-sized saucepan. Then pour in the water, sugar and lemon juice and let the sauce simmer for about 8 minutes, until the strawberries and rhubarb are soft.
Next, pour the sauce through a fine sieve. Save the juice and pour into a glass or metal pan measuring 12 x 8” (30 x 20 cm).
Put the mixture into the freezer for about 2-3 hours.
Crush the nearly frozen juice mixture with a fork to form chunky ice crystals.
Next, pour about 1 tbsp of Champagne into cocktail glasses and place the ice crystals on top.
Lastly, pass long cocktail sticks through the marshmallows and blowtorch them, so they soften and color nicely.
Place the marshmallows on the side of the glasses and decorate with strawberries and edible pink rose petals.