Leek and Swiss Chard Tart

Ingredients

• 2 tablespoons extra-virgin olive oil
• 4 leeks, white and pale-green parts only, thinly sliced and rinsed well (4 to 5 cups)
• 1/4 cup dry white wine
• 1 bunch Swiss chard, stems and ribs discarded, leaves shredded (4 cups)
• 1 tablespoon unsalted butter
• 2 tablespoons all-purpose flour, plus more for surface
• 1/3 cup whole milk, warmed
• 2 tablespoons Dijon mustard
• 1 large egg, lightly beaten
• Coarse salt and freshly ground pepper
• 1 cup grated Gruyere cheese
• Pate brisee