Caramelize the onions by sautéing them until golden brown. In the same pan, sauté the mushrooms until golden and all liquid evaporates. Season with salt and pepper. Set aside.
Combine eggs, dairy free cooking cream, salt, nutmeg, and pepper in a bowl, whisking thoroughly. Spread the sauteed mushroom and onion evenly over the pre-baked crust, then pour the egg mixture on top.
For a large, thick quiche (20 x 5 cm or 8 x 2 in), bake at 145°C (293°F) for 25 minutes, then adjust the temperature to 170°C (338°F) and continue baking for another 15 minutes.
For thinner quiche, bake at 170°C (338°F) for 35 to 40 minutes.
Let the quiche cool for 15-30 minutes before slicing.