Preheat oven to 425F
In a large skillet over medium-high heat, cook the bacon until crisp and brown, turning it frequently.
Transfer to paper towels and drain. Then chop and reserve. Reserve the bacon drippings.
Return the skillet to the fire, reduce the heat to medium
Add 1 tbsp of the drippings, then add the onions and cook, occasionally stirring occasionally, until golden brown and caramelized about 25 minutes.
Transfer the onions to a large mixing bowl and set aside
Add 1 tbsp of the drippings,
Increase the heat to medium-high. Add the mushrooms and cook, stirring occasionally, until they have released their water and begin to brown, about 6 to 8 minutes.
Add the onions to the pan, and stir in the mustard and thyme.
Season well with salt and pepper, and cook for one minute
Remove the pan from heat and set aside until needed
In a large bowl, whisk the cream, milk, cornstarch, salt, pepper, and eggs until well combined.
Lube the pie plate/tart pan well with oil
Carefully roll the puff pastry out onto the pie plate. Trim the pastry to the edge of the pie plate and use extra to fill in any holes.
Use a fork to poke several holes in the bottom of the pastry
Add bacon and cheese to the bottom of the pastry, and reserve a bit of cheese for the top.
Add the mushrooms and onions to the pastry
Pour the custard over and jiggle the pie pan to settle and remove air bubbles
Sprinkle with reserved cheese and any additional seasoning
Bake at 425 for 25 minutes or until fully set on top with no jiggle.
Rest for 15 minutes.
Serve in large wedges with a salad or fruit cup.