Place flour, sugar, cornmeal, and salt in a food processor and pulse to combine.
Add in the chilled butter and pulse a few more times, until the mixture has chunks of butter about the size of a pea.
Sprinkle in 2 – 3 tablespoons ice water and pulse until the dough holds together when squeezed.
If needed, add in another tablespoon of water.
Form the dough into a disk shape, cover the disk with plastic wrap, and place in the refrigerate for half an hour.
Make streusel topping-
Put the flour, sugar, and tarragon in the food processor, and pulse to combine.
Add in the butter and pulse until crumbly. Put the combined mixture into a small bowl and set aside int the refrigerator for now.
Preheat your oven to 400°F. In a large mixing bowl, combine the plums and sugar, then set aside.
Take the pie dough out of the refrigerator and roll it out on a flat, floured surface, rolling it out to fit in an 11″ x 7″ rectangular tart pan (or a 9-inch round tart pan).
Gently place the dough into the tart pan and coax it into the corners, using your fingers. Use a rolling pin over the top edges of the pan to remove any excess dough. Drain the plum/sugar mixture in a sieve and return the mixture to the bowl – discard the drained liquids. Add in salt and flour, and gently toss until the plums are coated evenly.
Shingle plum slices into prepared pan. Scatter streusel topping over the top. Place the tart on a rimmed baking sheet and bake for 40 to 45 minutes or until golden brown. Cool 30 minutes before serving your Plum Tart with Tarragon Streusel.