Zest and juice the lemon.
In a medium saucepan, combine blueberries, sugar and 1/4 cup of water. Bring to a simmer over medium-low heat, stirring frequently. Simmer until blueberries pop or burst easily with light pressure from the back of a spoon (5-10 minutes).
Combine corn starch with remaining 1/4 cup of water. When most of the blueberries have burst, drizzle the corn starch and water mixture over the syrup, stirring constantly.
Continue to simmer until filling thickens. The consistency should be thicker than maple syrup but not as thick as a fruit jam. Add half of the lemon zest, half of the salt and the almond extract (if using) to the filling and stir to combine.
Remove from heat and allow to cool completely (2-3 hours or overnight in the refrigerator).
Preheat oven to 400°.
Combine ricotta cheese, honey, remaining 1/4 teaspoon salt and 2 tablespoons of lemon juice. Set aside.
Roll out pie crust into a 12" circle on a floured surface. Transfer crust to a cookie sheet that has been lined with parchment paper.
Sprinkle a few of the almonds and the remaining lemon zest around the inner 9" circle of the crust (staying away from the outer edges). Drop the ricotta mixture by spoonfuls around the inner circle and gently spread to cover, leaving the outer 2"-3" of crust bare. See video in text above.
Spread a thick layer of blueberry filling over the top of the ricotta mixture. Any remaining filling can be used as a topping for pancakes or waffles or stirred into oatmeal or yogurt.
Gently lift an outside section of the crust and fold it over onto the blueberry filling. Working around the crust, continue to make folds that slightly overlap the previous fold until filling is enclosed. Gently press on the folds to "seal".
Whisk together the lightly beaten egg and a tablespoon of water and brush a thin layer all around the crust. Sprinkle crust with almonds and sugar.
Bake for 10 minutes. Reduce heat to 350° and bake for an additional 10 minutes, or until crust is golden brown and filling begins to bubble.
Remove from oven and allow to cool for 30 minutes before serving.