Step 1Combine the rhubarb, strawberries and sugar in a small saucepan over low heat until the fruit softens and breaks down, stirring occasionally, about 15 minutes. Stir the cornstarch and water together in a small bowl. Add to the fruit and continue cooking on low heat, stirring often, until the fruit thickens, about 5 more minutes. Cool completely and store covered in an airtight container in the refrigerator. It will stay fresh up to a week.
Step 1Beat the butter and sugar in the bowl of an electric mixer until light and fluffy. Scrape down the sides of the bowl and add the egg and vanilla; beat until smooth.
Step 2Sift the flour and salt together then stir in the cardamom. Add the flour to the butter mixture on low speed just until combined. Stir in the almonds.
Step 3Divide the dough into 4 pieces and flatten into rectangles. Wrap each in plastic wrap and refrigerate until firm, at least 1 hour.
Step 4Heat the oven to 375 degrees; grease 2 baking sheets. Remove 1 rectangle of dough at a time.
Step 5Roll out the dough on a very lightly floured work surface to 1/8-inch thick. Using a 2 1/2-inch fluted cookie cutter, cut out as many cookies as you can. Then, using a 1-inch fluted cookie cutter, cut a hole in the center of half of the fluted cookies (these will become the tops of the sandwich cookies). Carefully place all cookies on the baking sheets.
Step 6Make the egg wash by stirring the beaten egg with water. Brush just the top cookies (with the hole) with the egg wash. Sprinkle the sparkling sugar over the egg-washed cookies. Bake until a pale golden brown, 10 to 11 minutes. Cool on a rack.
Step 7Continue with the remaining dough until all the cookies are baked and you have an equal number of tops and bottoms.
Step 8Place 1 teaspoon of jam in the center of each of the bottom cookies, spreading to within 1/2-inch of the edge. Place a top cookie over each and press gently.