In a food processor, blender or bullet puree strawberries and pineapple.
Transfer fruit puree to a stainless steel saucepan over medium-high heat. DO NOT BOIL.
Add lemon juice, sugar, and stir to combine.
Using a thermometer heat to 190 degrees Fahrenheit, stirring occasionally. Because this recipe has sugar it will get to temperature rather quickly so keep your eye on the thermometer. Remove from heat.
Cool slightly before ladling into jars.
Ladle mixture into hot pint jars or quarts, leaving 1/4 inch headspace.
Wipe rims clean, add hot canning lids, and tighten rings just finger tight. Process in water bath canner at a boil for 15 minutes, adjusting for altitude.
To reconstitute, mix one part concentrate with one part water, tonic water or ginger ale. Adjust concentrate to suit your taste.
For a smoother drink, strain to remove pulp or seeds before heating.