• Add 3 cups sliced rhubarb, 1 cup sliced strawberries, and 1/2 cup water to a medium saucepan. Cover and cook over medium heat until the rhubarb has mostly broken down and the strawberries have softened, about 10 minutes.
• Blend using an immersion blender (or carefully transfer the mixture to a blender) until smooth. If desired, strain the blended mixture through a fine mesh strainer to remove seeds and pulp, stirring and pressing the mixture against the mesh (note: this process will take quite awhile as it strains slowly).
• Prepare a double boiler, or set a glass or stainless steel bowl over a small saucepan. Add about an inch or two of water to the saucepan, ensuring that the water doesn't touch the bottom of the bowl. Bring the water to a boil, then reduce to a simmer.
• In the bowl of the double boiler, whisk together the rhubarb mixture, 6 large egg yolks, 3/4 cup granulated sugar, and 1/4 teaspoon salt. Continue cooking it in the double boiler, whisking constantly to prevent curdling, until it becomes thick enough to coat a spoon, and reaches 170°F, about 12-15 minutes. If it still doesn't thicken, turn up the heat slightly and continue whisking.
• Carefully remove the bowl from the double-boiler (use oven mitts) and stir in the cubed butter until melted. Cover the Strawberry Rhubarb Curd directly with plastic wrap (or transfer to a quart jar) and refrigerate until chilled (about 2 hours). Use the curd as a filling for cakes or pies, serve with yogurt or ice cream, or simply spread it on toast.

Optional Thickening

• If you plan to use this curd as a filling for cakes, tarts, or pies, I recommend adding some extra thickening to it (this is not needed if you plan to use it as a sauce, topping, or spread). You can do this step immediately after adding the butter, or after it's already been chilled.
• Heat the curd in a medium saucepan over medium heat, stirring almost constantly to prevent scorching. In the meantime, stir 1-2 Tablespoons cornstarch with an equal amount of cold water. When the strawberry rhubarb curd starts to bubble, stir this cornstarch mixture into the curd. Keep cooking until the curd thickens, about 2-3 more minutes. Remove from the heat, and use as desired. (Note that the curd will thicken even more as it cools).