Quarter strawberries, and bring strawberries, water, sugar, and ginger to boil in a medium saucepan. Simmer for 10-15 minutes until the mixture is nice and fairly thick, coating the back of a wood spoon. In the last 5 minutes, stir in the fennel seeds. Cool for about 5-10 minutes.
Pour the strawberry mixture through a fine mesh strainer or cheesecloth to squeeze the last of the juice and color out of the berries. (You can pick out the larger ginger chunks and use the strawberries for jam.)
Combine the cooled syrup with lemon juice (or citric acid). Pour about 1-2 ounces into a bottom of a glass, then top with ice and 4-5 ounces of sparkling water or club soda. Garnish with fresh strawberries or mint.
Notes
This makes about 2 cups of syrup so you can use more or less in each drink. This can make 10-16 drinks. Feel free to adjust the simple syrup amounts to your liking. Generally I find 1-2 ounces of simple syrup with 4-5 ounces of sparkling water is a light and refreshing drink. You can also make this more concentrated, using 1 ounce of simple syrup in a coupe glass and a small splash of sparkling water on top.
Storage - Store the strawberry syrup in a mason jar in the fridge for up to 5 days.
Citrus adjustment - Citric acid alternative - Use 1 1/2 teaspoons of citric acid in place of lemon juice. Or leave out entirely if sensitive.