3/4 cup ricotta cheese

3/4 cup Parmesan cheese, divided

1/4 cup chopped chives or green onions, plus more for sprinkling

1/4 cup heavy cream

Preheat the oven to 350 F.

To make the dough, place the flour and salt in a food processor and pulse. Drop in the butter and pulse until just barely the texture of oatmeal. Add the ice water slowly and pulse once or twice, until the dough clumps together. Turn onto a floured board and form into a ball. Wrap in plastic wrap and chill in refrigerator for at least 30 minutes.

Meanwhile, peel and remove the root ends from the onions. Slice them in quarters and place in a pie dish or small roaster. Pour 2 tablespoons of olive oil, the vinegar, 1 teaspoon salt and 1/4 teaspoon pepper over top. Roast until the onions are soft and begin to color but still hold their shape, about 45 minutes. Remove from the oven and cool.

While the onions are roasting, cook the mushrooms. Over medium heat, melt the butter. Add mushrooms, 1/4 teaspoon salt and 1/4 teaspoon pepper, and saute for about 5 minutes until golden. Remove from the heat.

When pastry dough has chilled, roll it out onto a lightly floured surface to a circle roughly 12 inches in diameter. Gently transfer to a tart ring, and fit into the ring evenly. Pierce bottom with a fork and return to refrigerator to chill another 20 to 30 minutes.

Separate the egg and transfer the yolk to a small bowl; reserve the egg white for the topping. Mix the egg yolk, ricotta, remaining 1 tablespoon olive oil, 1/2 cup of Parmesan, and chives. Remove the tart shell from refrigerator. Spread the ricotta mixture evenly across the bottom of the tart shell. Nestle the onions across ricotta base and scatter the mushrooms in between onions.

In another small bowl, beat the reserved egg white, heavy cream and the remaining 1/4 cup Parmesan. Pour mixture evenly over top of onion and mushroom filling.

Cover the tart with foil and bake at 350 F for 30 minutes. Remove the foil and bake another 15 minutes, until top is bubbling and crust is slightly brown, but not too dark.

Sprinkle chives over top before servings. Serve warm or at room temperature.

(Sara Kate Gillingham is founding editor of TheKitchn.com, a nationally known blog for people who love food and home cooking. Submit any comments or questions to kitchn@apartmenttherapy.com.)