1. Crust: Pulse flour, oats, and salt in a food processor to combine. Add butter, cream cheese, and egg yolk; pulse until dough just holds together, 15 to 20 seconds.

2. Turn dough out onto a piece of plastic wrap and flatten into a disk; wrap in plastic. Refrigerate at least 1 hour or overnight.

3. Filling: In a large skillet, heat 1 tablespoon oil over medium. Add chard stems, onion, and a pinch of salt. Cook, stirring, until stems are soft and slightly brown, 8 to 10 minutes. Add vinegar and cook, stirring, until liquid is almost evaporated, about 2 minutes. Season with salt and pepper. Stir in raisins. Transfer mixture to a nonreactive bowl.

4. Heat remaining 1 tablespoon oil in same skillet over medium-high. Add anchovies, if using; cook, stirring frequently, 1 minute. Stir in thyme. Add chard leaves and cook until slightly wilted, about 1 minute. Season with salt and pepper.

5. In the bowl of a mixer fitted with the paddle attachment, beat both cheeses with 2 tablespoons cream until smooth, about 1 minute. Stir in nutmeg; season with salt and pepper.

6. Preheat oven to 375 degrees. On a large sheet of parchment, roll out dough to a 1/8-inch-thick round, about 15 inches in diameter. Arrange onion mixture evenly over dough, leaving a 3-inch border. Spread ricotta mixture over onion mixture; top with chard mixture. Fold edges of dough over and gently press down to seal. Transfer tart (still on parchment) to a baking sheet. Refrigerate 15 minutes.

7. In a small bowl, beat together egg yolk and remaining 1 tablespoon cream. Brush exposed dough with egg wash. Bake until crust is golden, 40 to 45 minutes. Serve warm or at room temperature.