Preheat your oven to 350°F. In a bowl, combine the onions, olive oil, rosemary, sage, thyme, and salt. Toss well to coat. Spread the mixture onto a rimmed baking sheet in an even layer. Roast for about 45 minutes or until the onions are soft and beginning to brown slightly but still hold their shape. Remove from oven and allow to cool on a wire rack. Increase oven temperature to 375°F.

In a food processor, pulse the flour and salt to blend. Add the cold butter and pulse until the texture resembles coarse crumbs or small peas. While pulsing, add 3 tablespoons of ice water through the top. Pulse until the dough starts to clump. Add more water, 1 tablespoon at a time, if needed. Turn the dough onto a floured surface and form it into a flat disc. Roll out to a 13-inch circle and place it into a 10-inch tart pan, pressing gently into the edges. Trim any excess dough, prick the bottom with a fork, and chill for 30 minutes.

In a small bowl, whisk the ricotta, egg yolk, and olive oil until smooth. Stir in the crème fraîche and season lightly with salt and pepper.

In another bowl, whisk the egg with the flour and a pinch of salt. In a small saucepan, heat the cream and milk just until hot, not boiling. Slowly whisk the warm milk mixture into the egg mixture to create a smooth custard. Stir in the crème fraîche.

Spread the cheese filling evenly over the chilled tart crust. Scatter about three-quarters of the roasted onions over the filling. Pour the custard evenly over the top. Arrange the cheese slices over the custard and dot with the remaining onions.

Place the tart in the oven and bake for 30 minutes, rotating the pan halfway through. Loosely tent with foil and bake for another 15 minutes. If the crust or top needs additional browning, increase the heat to 400°F and bake uncovered for another 10 to 15 minutes.

Let the tart rest in the pan on a wire rack for at least 5 minutes before slicing. Serve warm or at room temperature.