First, make the pastry cream! This will need to chill in the fridge for at least 2 hours. In a small saucepan, heat your whole milk and the heavy cream and 2 Tbsp of the sugar and heat over medium low, stirring frequently, until it comes to a simmer. While the milk mixture is heating, whisk together the remaining 2 Tbsp of sugar, cornstarch, salt, and egg yolks in a medium bowl until thick and smooth.
Finally, once milk is simmering, remove from heat, and, whisking constantly, pour 1/2 cup of the mixture slowly into the egg yolk bowl to temper your egg yolks. Once tempered, put the saucepan back over low heat and whisk in the tempered egg mixture. Whisk constantly as the saucepan mixture continues to thicken over low heat. Once thick, remove from heat and whisk in the 2 Tbsp salted butter and vanilla extract until smooth, then scoop the pastry cream into a bowl, place plastic wrap directly on top of the pastry cream, and refrigerate for at least 2-3 hours (can make ahead if you'd like).
Next, make your crust! Preheat the oven to 325° Fahrenheit and lightly grease your 10-inch tart pan with cooking spray then set aside. In a large bowl, combine all of your tart crust ingredients, stirring until it resembles wet sand. Dump crust into tart pan and press the crust into the base and up the sides so that you have an even layer lining the entire pan (patting down the edges). Bake for 20-25 minutes or until drier to the touch. Let crust cool completely before filling.
Then, make your roasted fruit. Line a cookie sheet with a larger piece of parchment paper and raise the temperature of the oven to 350° Fahrenheit. In a large bowl, combine your peeled and chopped rhubarb, your hulled and sliced strawberries, and all other ingredients until the fruit is coated in the other ingredients. Dump fruit onto the prepared cookie sheet and spread so that you have a single layer of fruit in the juice/coating. Bake for 45 minutes total, carefully spooning some of the pan juices over fruit every 15 minutes. Let fruit cool completely before topping your tart with it.
Once everything is cooled/chilled (you can make all three components ahead of time), stir the pastry cream to loosen, then spread the pastry cream into the tart crust, smoothing to the edges. Finally, remove the tart from the pan and plate, and carefully spoon the cooled roasted fruit (and all of the extra juices in the pan!) on top of the pastry cream, and then serve! Keep refrigerated until serving (the pastry cream needs to stay chilled).