In a food processor, blend the flour and salt. Add the butter and pulse a few times, until it forms pea-sized pieces. Add the yogurt and water. Pulse just until the dough starts coming together. Add more water, as needed. Remove the dough from the food processor and form into a rectangle with your hands.
On a floured work surface, roll the dough into a 15 x 12-inch (38 x 30 cm) rectangle. Line a small 13 x 9-inch (33 x 23 cm) baking sheet with the dough. Refrigerate for 30 minutes.
With one rack in the middle of the oven and a second rack in the lowest position, preheat the oven to 400°F (200°C).
Prick the dough all over with a fork. Cover the dough with a sheet of foil and fill with pie weights or dry beans. Refrigerate.
Place the onions on one half of a non-stick baking sheet. Drizzle with the balsamic vinegar and half of the oil. Place the mushrooms on the other half of the baking sheet. Drizzle with the remaining oil. Season the vegetables with salt and pepper.
Roast the vegetables on the middle rack and the dough on the lower rack of the oven for 20 minutes. Once out of the oven, add the spinach to the mushrooms and toss to combine, without touching the onions. Remove the pie weights and the foil from the dough. Continue baking both sheets for 5 minutes or until the vegetables and crust are golden. Set aside.
In a bowl, whisk together the eggs and cream. Add the cheese. Season with salt and pepper. Mix well. Pour the egg mixture into the crust and top with the spinach-mushroom mixture. Gently nestle the onion rounds among the other vegetables.
Bake the quiche on the middle rack of the oven for 20 to 25 minutes or until the filling has set and the crust is nicely golden.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.