Using a medium-sized mixing bowl, stir together the cut rhubarb, strawberries and ½ cup granulated sugar. Stir to completely combine. Cover the bowl and allow the rhubarb mixture to sit at room temperature for 30 minutes. Rhubarb and strawberries release a lot of liquid. The sugar will help to draw out quite a bit of the liquid.

Using a colander, drain the liquid that has been released by the rhubarb and strawberries. Once all the liquid has been drained, return the rhubarb and strawberries to the mixing bowl.

Add the remaining granulated sugar, cornstarch, lemon zest and lemon juice. Stir to completely combine. Set it aside.

Preheat the oven to 375°F.

Dust your work surface with flour, and unroll the 1st pie crust.

Using a 4-inch round cookie cutter, cut out the mini pie crusts. You should be able to get at least 5 crusts from each roll of pie crust. You can reserve the leftover crust and roll it out if needed. Repeat until you have cut out 12 crusts.

Generously spray a standard muffin pan with nonstick spray.

Evenly press each crust into the muffin cups.

Roll out the third pie crust. Using a pizza cutter, cut out 60 pieces, ¼-inch x 3¼-inch strips. This will be the lattice topping (five strips per pie).

Fill the pie cups ⅔ full.

Using the five pie crust strips, lay three strips on top of the pie.

Fold the end pieces back a little more than halfway, leaving the middle strip down.

Lay the fourth strip on top of the filling perpendicular to the three strips. Fold down the top two folded strips.

Fold up the middle strip.

Lay the fifth strip under the fourth strip. Fold down the middle strip.

Gentle press the edges of the crust and lattice, to ensure they stay in place during baking.

Using a small mixing bowl, whisk together the egg and water. Using a pastry brush, generously brush the egg wash on top of the pies.