How to Make It
In a bowl, beat the butter with the milk, sugar and egg yolk until smooth. Add the flour and beat at low speed until the dough starts to come together. Pat the dough into a disk and wrap in plastic. Refrigerate for 1 hour.
On a lightly floured surface, using a lightly floured rolling pin, roll out the dough to a 12-inch round. Ease it into a fluted 9-inch tart pan with a removable bottom; trim the overhang. Refrigerate until firm, 1 hour.
Preheat the oven to 375°. Line the dough with parchment paper and fill with pie weights. Bake for about 30 minutes, until set. Remove the weights and parchment paper and bake until golden brown, 8 to 10 minutes longer. Transfer the tart shell to a rack and let cool to room temperature, about 1 hour.
In a large saucepan, heat 2 tablespoons of the olive oil. Add the onion and cook over moderate heat until softened. Add the eggplant and 3 tablespoons of the olive oil, season with salt and pepper and cook, stirring occasionally, until starting to soften, about 3 minutes. Add the red peppers, zucchini and the remaining 3 tablespoons of olive oil and cook until softened, 7 minutes. Stir in the tomatoes and oregano and cook until the vegetables are tender and the sauce is thick, about 20 minutes. Stir in the vinegar. Season with salt and pepper and let cool.
Spoon the ratatouille into the tart shell and serve.