Combine the strawberries, rhubarb, 1/4 cup of sugar, lemon juice and salt in a 3-quart saucepan. Place the pan over medium heat and cook the fruit until the rhubarb turns soft. The mixture should be very thick. If it gets too dry before the rhubarb is tender add 2 tablespoons of water. Once the rhubarb is tender, puree the mixture with a stick blender until all the fruit is broken down.
Place a bowl with a coarse strainer near the stove. In a separate bowl combine the eggs and the remaining sugar (1/4 cup) and whisk until the egg yolks are lighter in color, about 1 minute. Slowly drizzle half the warm strawberry-rhubarb puree into the eggs, whisking the entire time you’re adding the puree. Continue to whisk until the eggs and fruit are well combined. Scrap the egg mixture into the pot with the rest of the fruit puree. Return the pot to the stove and continuously whisk over medium heat until the curd reaches a temperature of 175 degrees F.
Remove the curd from the heat and whisk in the butter one tablespoon at a time. Scrap the curd into the strainer and use the back of a tablespoon to push the puree through into the bowl below. Save the solids that remain for toast or biscuits.
You should have 1-1/4 cups of strawberry-rhubarb curd. Place 1/4 cup in each of the tart shells. Put a cooling rack on top of a half baking sheet and place the filled tarts on top.
Bake for 15 minutes until the curd is set. You know it is set, when you lightly touch the curd with your finger and no imprint is left.
Remove the tarts from the oven and allow to cool to room temperature. Top each tart with whipped mascarpone or whipped cream, fresh fruit, and enjoy.