With a slotted spoon, scoop thawed strawberries into a medium pot and mash with a potato masher or fork. Pour reserved juices from thawed strawberries into a measuring cup. Add enough water on top of strawberry juice to equal 1 ½ cups of total liquid. Add liquid to pot with strawberries and turn heat to high. Bring to a boil, stirring occasionally. Reduce heat to low and simmer for 15 minutes.
Set a fine mesh strainer over a large bowl and strain strawberry juice into bowl, pressing on strawberry pulp to ensure that all juice has been extracted. Discard pulp. Return juice to pot, stir in sugar until dissolved, and bring to a boil again. Reduce heat to low and simmer for 5 minutes, stirring occasionally and skimming off any foam that may appear.
Remove syrup from heat and allow to cool to room temperature. Store, covered, in the refrigerator.
TO MAKE STRAWBERRY SODA:
Measure one part chilled strawberry syrup to two parts chilled seltzer. Taste and increase syrup or seltzer, depending on your preferences. Pour over ice or serve as-is.
You may use fresh, very ripe strawberries instead of frozen strawberries if that's what you have available. Just hull and dice them before mashing, and use a full 1 ½ cups of water in which to cook them.
If desired, strawberry soda may be enhanced with a squeeze of fresh lime juice.
Strawberry syrup can also be used over vanilla ice cream, pound cake, waffles, pancakes, French toast, etc.