Preheat oven to 375F and line baking sheet with parchment.
Place rhubarb in a large bowl. Add sugar, cornstarch, cinnamon, nutmeg, salt, lemon juice, and vanilla. Mix well until combined.
Roll out pie dough to an 11" disc and place on parchment lined baking sheet. Place rhubarb in the middle of the pie crust leaving a 2" border. Fold dough over and pinch to enclose the rhubarb, leaving the center open.
Brush crust with egg wash and sprinkle with coarse sugar and sliced almonds if desired. Chill for 15mins. Add small pats of the 1 Tbsp of butter on top of the rhubarb.
Bake for 35-40mins, until crust is golden and juices are bubbling.
Cool on sheet for 10mins. Slide off with parchment onto wire rack to cool completely.
Store in fridge. Serve cold or reheated.