Blueberries are available year round, in many locales. While it is best to use fresh berries in the filling, you can use frozen berries or a combination of the two. Frozen berries will yield a syrupy, sauce-like compote that you will need to strain through a fine mesh strainer, then add back just enough liquid to create a jammy filling. There are any number of high-quality jams available that are perfectly agreeable in tarts, cookies, or as a topping on blintzes. The best scenario, however, is to receive a gorgeous jar of blueberry jam from a friend. Thank you, Frances.
Blueberry filling/topping: Makes 2 cups
3 cups blueberries (preferably fresh; if using frozen, thaw briefly on paper towels to remove excess ice)
½ cup granulated sugar (if they are terribly tart, add an additional tablespoon or two of sugar)
1 teaspoon lemon juice
2 teaspoons lemon zest
2 teaspoons cornstarch
½ teaspoon vanilla extract (I use Jones and Company)
Place 2 cups of blueberries in a heavy bottomed, medium saucepan. Add sugar, lemon juice, lemon zest, and cornstarch, tossing with a rubber spatula to coat the berries. Simmer the mixture over medium heat, stirring occasionally, until the liquid thickens and the berries begin to burst, about 10-15 minutes. Remove from the heat and stir in the remaining 1 cup of blueberries. Cook an additional 5 minutes, until the liquid reduces. (The filling should register 228 degrees F on a candy thermometer.) Strain excess liquid. The mixture will thicken as it cools. (Save the excess liquid in case you want to thin down the berry mixture a bit.) Add vanilla extract to the berries. Cool completely before using. If not using right away, cool, place in a clean glass jar, cover and refrigerate. The filling (and extra syrup that you poured off) will keep refrigerated for up to one week.