Set a large skillet over medium low heat. Spray or gently drizzle with oil (or butter).
While the pan is heating, prepare the batter in a medium, wide bottomed bowl. Add the flaxseed and cornstarch to the bowl. Slowly add in the milk, stopping frequently to whisk in the cornstarch. Once all clumps of cornstarch have been whisked, finish adding the milk.
Add vanilla, cinnamon, and salt (if desired) to the milk mixture and whisk to combine.
Quickly tip each side of the bread in the milk mixture, removing quickly. Allow the excess to drip back into the bowl, then transfer the dipped bread to the heated skillet.
Cook the bread about 3-5 minutes on each side, until it reaches your preferred level of browning. Flip each piece and continue cooking for another 3-5 minutes. The pan should sizzle gently, but not make a loud commotion. If the pan is sputtering, or the toast (or oil) is browning quickly, turn down the heat slightly. Alternatively, if the toast does not sizzle when it hits the pan, you may need to turn the heat up.
Once cooked, remove from the heat. Once toast can be handled safely, slice into the top of the toast with a small knife, creating a pocket for the filling.
Using a knife, spoon, piping bag, or zip top bag with a corner cut, fill each pocket with your filling of choice before serving.
Serve french toast warm topped with syrup, fruit, whipped topping, cinnamon, or whatever your heart desires!