In a medium saucepan over medium-low heat, combine rhubarb and strawberries with 1/3 cup sugar, 1 tablespoon lemon juice and water. Cook 15 minutes, stirring occasionally, until rhubarb falls apart. Remove from heat. Cool completely, then puree in a food processor or blender until smooth. Reserve 1 cup puree.

Fill a medium saucepan halfway with water; set over medium-low heat. When water is close to a simmer, top saucepan with another medium saucepan (to create a double boiler). Add egg yolks, remaining sugar and salt to saucepan; whisk to combine. Add reserved rhubarb puree and continue to cook, whisking constantly, 10 minutes until an instant-read thermometer inserted in the mixture reads 160°F (run your finger along the back of a spoon; if the path remains, it’s ready). Remove from heat; stir in cubed butter and grenadine for color, if desired.

Pour curd into a small bowl; cover top of curd directly with plastic wrap. Chill in refrigerator at least 4 hours to cool and thicken.