For the meringues: position one rack in the lower third of the oven and one rack in the upper third, and preheat to 200°F. Line 2 large baking sheets with parchment paper.
In a small bowl, combine 2 teaspoons of the sugar and the cardamom and mix well. In the bowl of an electric mixer, place the egg whites. Using medium speed, beat until the whites are frothy. Add the cream of tartar and beat on medium-high until soft peaks form, about 2 minutes. Gradually add the remaining 1 cup of sugar 1 tablespoon at a time, beating constantly and taking about 3 minutes. Sprinkle on the cardamom mixture and add the vanilla and salt. Beat until very stiff and glossy and the meringue holds a peak when the beater is lifted straight up, 1 to 2 minutes longer.
Dab a bit of the meringue on the backside of the parchment at the corners to secure the parchment to the baking sheet. Using a rubber spatula, immediately drop the meringue in 8 mounds (about half-cupful) onto the baking sheets. Using the back of a metal spoon, gently create a deep, wide indentation in the center of each.
Bake meringues until dry and crisp and can be lifted easily from the parchment, about 2 hours. Turn off the heat and let dry completely in the closed oven, at least 2 hours longer.
For the lemon curd: place a fine sieve over a medium bowl. In a heavy medium saucepan, combine the butter, ½ cup of the sugar, lemon juice and zest. Stir over medium-high heat until the sugar dissolves and the mixture just comes to a simmer. Whisk the eggs in another medium bowl to blend. Gradually whisk in the hot lemon mixture. Return the mixture to the same saucepan. Cook over medium-low heat, stirring constantly, until the curd thickens (do not boil). Immediately pour the curd through the sieve, pressing all the curd through. Cool slightly. Press plastic wrap on the surface of the curd and refrigerate until cold.
Fold the Greek yogurt into the curd, or whip the cream to stiff peaks and fold the cream into the curd. Cover and refrigerate until assembling the desserts. Combine the strawberries and blueberries in a medium bowl. Mix in the remaining ¼ cup sugar. Let stand at room temperature until juices form, about 30 minutes.
Just before serving, spoon about 3 tablespoons lemon curd into each meringue cup. Top each with berries and serve right away.