Begin by thawing your frozen raspberries, which will take around 30 minutes. Once softened, place them into a food processor and blend until smooth, about 1-2 minutes.

Next, pour the raspberry puree through a fine mesh strainer into a large mixing bowl. Use a spatula to push the liquid through while keeping the seeds behind.

In a large pot over medium heat, combine the freshly strained raspberry juice with freshly squeezed lemon juice and granulated sugar. Stir until the sugar dissolves completely.

Once the mixture is heated through, remove the pot from the stovetop and let it cool for about 10-15 minutes.

While the concentrate is cooling, sterilize your jars, lids, and bands by washing them in hot, soapy water and rinsing thoroughly. Then, place the jars in a boiling water bath for 10 minutes.

Using a ladle and funnel, fill each sterilized jar with the warm raspberry lemonade concentrate, leaving a 1/4-inch headspace at the top.

Wipe the rims of the jars with a clean cloth to remove any residue, then place the sterilized lids on top and screw on the rings just until snug.

Process the jars in a boiling water canner for 15 minutes, adjusting the time slightly for high altitudes.

After processing, carefully remove the jars from the canner and let them cool completely on a clean kitchen towel.

To enjoy, mix with water in a ratio of 1 part concentrate to 3 parts water, stir well, and serve over ice.