In a large bowl, stir together the rhubarb, strawberries, brown sugar, white sugar, cornstarch, cinnamon, lemon zest & salt so that the strawberries are evenly coated.
Roll out 1 pie disc so that it forms a circle about 12 inches in diameter. Carefully transfer to a 9-inch pie plate.
Spoon the pie filling into the pie plate using a slotted spoon. Do not add the juice at the bottom of the bowl (this will make your pie soggy).
Roll out the second pie disc. If making a double crust: Carefully transfer to the top of the pie. Trim excess crust from the edges, then fold them over, and crimp the edges. Use a sharp knife to cut make an X-shape in the center. If making a lattice-topped pie: After rolling out the second disc, cut it into strips about 1/2 - 3/4 inch thick using a pizza knife or sharp knife. Place 4-5 strips vertically on top of the pie, then weave strips through horizontally, alternating between going over & under. Cut the excess off the edges, then fold the edges over and crimp them with your fingers.
In a small bowl whisk together the egg and milk. Gently brush the top of the pie with the egg wash (there will be leftover), then sprinkle with sugar.
Place the pie on top of a cookie sheet, and put both in the oven. Bake in the preheated oven for 15 minutes, then turn the oven down to 375F degrees and continue baking for 50-60 minutes. Check the pie after both 15 & 30 minutes, if the top is getting too dark tent a piece of aluminum foil over top.
Cool for at least 3 hours before slicing and serving.
*Store pie for up to 8 hours at room temperature, then store in the fridge for up to 4 days. Baked pie can be frozen and thawed in the refrigerator.