Cut the vanilla bean in half length-wise. Scrape out the glorious caviar inside each half of the bean.
In a medium saucepan slowly heat the milk, vanilla bean, and sugar under low heat. Go ahead and throw in the empty vanilla pods in there too to get all the vanilla flavor out of the bean. Constantly stir the milk making sure it does not burn at the bottom. Once milk burns you have to start fresh, and that's just a waste of vanilla beans! Do not let the milk boil. Turn it off once you begin to see little bubbles forming around the edges. Take out the vanilla pods. Turn off heat from pan.
In a medium mixing bowl, crack the eggs and whisk. Whisk in the salt and cornstarch. Temper the egg mixture. Meaning, pour about 1/3 of the warm milk into the eggs and stir. Then pour the egg mixture into the saucepan back with the milk. This prevents the eggs from getting shocked by the heat and scrambling. Turn the low heat back on. Now we want to slowly simmer this custard so the cornstarch activates and can thicken the mixture. Again constantly stir and keep an eye on the simmer.
Once it comes to a simmer and begins to thicken you can turn it off and pour it in a mixing bowl as it will continue to cook. Take a tablespoon of butter and whisk it in until it melts. Repeat until you do all 4 tablespoons of butter. Place plastic wrap directly on top of the custard to prevent a film from forming and refrigerate for 2 hours.
After everything has cooled, it's time to assemble! Pour the chilled custard into the tart crust. Spread out evenly with a spatula. Place fresh raspberries one at a time on top of the custard covering the entire surface. Sprinkle blueberries in the gaps. Zest a lemon over the entire tart. This is where I would tell you to chill the tart for another couple hours before cutting into it but I didn't becuase I don't have strong will power. So I'll let you decide.