In a large frying pay, saute mushrooms and onion in oil until soft. Add spinach and stir until wilted. Season with salt and pepper and set aside.
Meanwhile, whisk together eggs and cream until smooth. Season with salt and pepper.
Grease a 9 x 13 baking dish with vegetable oil. Line bottom with cubed bread. Scatter mushroom spinach mixture on top and sprinkle with cheese. Pour egg mixture over top.
Allow to sit for at least 15 minutes, poking the bread back into the mixture if it rises.
Bake for 50 minutes or until custard is set. Turn on broiler and broil for a few minutes until the top is golden. Watch carefully!
Let stand 8-10 minutes, then serve.
Recipe by Life at Cloverhill at https://lifeatcloverhill.com/2015/12/mushroom-spinach-strata.html