Heat a large sauté pan over medium heat. Add oil, mushrooms, salt, pepper, leek, and garlic. Cook about four minutes, stirring often until mushrooms are soft, then add a splash of cognac if using. Cook an additional minute while stirring, then remove from heat. Add the tarragon and parsley and stir to incorporate. Taste to test seasoning, adding additional salt and pepper to your liking. Transfer to a plate and spread into a thin layer for a faster cool-down.
Meanwhile, in a small bowl stir to combine the ricotta and lemon zest. Spread a teaspoon or so of this mixture onto the bottom of each tart shell, using all of it up. Top with the cooled mushrooms, dividing evenly among the 12 tarts (roughly a heaping teaspoon in each).
Place tart shells onto a baking sheet and bake in preheated oven 20-25 minutes until pastry is cooked through.
After baking, carefully remove foil tart wrappers and top each tart with a nub of brie to serve. Side note: I prefer my brie un-melted, so I let the tarts cool to room temperature before topping because I get more cheese-y flavour that way. But if melted brie is your jam, no need to wait for the cool-down.