To a saucepan, add the strawberries, lemon juice, and granulated sweetener. Mix well to combine. Cover and leave the mixture at room temperature overnight.
Step 2
Place the saucepan over low heat for about 4 minutes to gently warm the strawberries without boiling the mixture. Continuously stir until the granulated sweetener dissolves entirely and the mixture releases a slightly thick deep red strawberry juice. Turn off the heat and set the saucepan aside.
Step 3
Pour out the strawberry juice through a sieve, pressing with the back of a spoon to release as much juice as possible. You should be left with slightly chunky strawberry pieces in the sieve and slightly thick strawberry juice at the bottom. Funnel the cordial into a sterilized bottle and seal.
Step 4
To serve, dilute 2-3 Tbsp of the cordial with 1 cup of sparkling water. Pour over ice and garnish with a slice of lemon and fresh mint leaves. Store any leftover cordial in an airtight bottle in the refrigerator for 3-5 days.