In a large saucepan, cover rhubarb chunks and strawberries with water. Bring to a boil over medium-high heat, stirring frequently. Reduce the heat to low and simmer gently until the rhubarb has softened.
Place a fine-mesh sieve over a bowl. Add rhubarb-strawberry mixture and allow liquid to drain out. Add gentle pressure. Add the sugar to the warm mixture and stir until the sugar has dissolved.
Transfer to a glass jar. Cover and refrigerate for up to 1 month.
Sparkling strawberry rhubarb cocktail
Add syrup and lemon juice to glass. Top with bubbly champagne or prosecco.
Garnish with a rhubarb ribbon and sliced strawberry and serve.