Preheat your oven to 350°F (175°C).

In a medium skillet, heat a splash of oil over medium heat. Add the sliced mushrooms and diced onion, and sauté for about 5-7 minutes until tender.

In a mixing bowl, combine flour and salt. In a separate bowl, whisk together the oil and cold water until smooth. Gradually add to the flour, mixing gently until a dough forms. Let rest for a few minutes.

Divide the dough into portions and flatten into disks. Press into mini tart pans, ensuring even coverage, and prick the bottom with a fork.

In a large bowl, beat together eggs and cream until well combined. Season with salt and pepper if desired.

Distribute the sautéed mixture among the crusts and sprinkle with Swiss cheese. Pour the egg mixture on top, filling each about three-quarters full.

Bake for 30-35 minutes, until edges are golden and filling is set but still slightly jiggly.

Cool in pans for about 5 minutes, then transfer to a serving platter. Serve warm.