The sweet-tart citrus filling and fresh blueberries in this tart taste like spring. Notes: You can make the crust a day ahead and store it airtight at room temperature, but fill it on the day you'll be serving.
Preheat oven to 325°.
Make matzo meal crust: In a food processor, whirl matzo meal, sugar, and margarine until well mixed. Add egg and 3 tbsp. water; whirl until dough holds together. Dip fingers in more matzo meal and press the soft dough over the bottom and sides of a 9-in. tart pan with a removable rim.
Bake until crust is darker brown and feels firm when pressed in the center, 30 to 40 minutes.
Meanwhile, make filling: In a small saucepan, mix sugar with potato starch. Stir in zest, orange juice, and lemon juice. Stir over high heat until boiling. Remove from heat and whisk in eggs. Return to high heat and stir just until mixture bubbles. Let cool at least 10 minutes.
Spread warm filling in tart shell. Scatter blueberries on top. Serve tart warm or cool, cut into wedges.