Prepare your tart base, roll it out and place it in a tart dish.
Clean your mushrooms and cut them into strips.
Slice the shallot and cut the garlic into very thin slices.
In a pan, heat the butter with a drizzle of oil. Brown the garlic and shallot for 2 minutes over medium heat.Then add the mushrooms and stir as little as possible otherwise they will release a lot of water.Heat for 10 to 15 minutes so that the mushrooms start to brown on some sides.Salt and pepper then place them on your tart base.
Preheat your oven to 180°C.
In a bowl, beat together the egg, whole grain mustard and cream then pour over the mushrooms.
Scrape the cheddar on top and bake for 40 to 50 minutes, until the tart is golden brown.
Notes
If the mushrooms release a lot of water, drain it so they can brown well.