Clean the Leeks: Trim off the roots and the top dark green parts which are tough and not as pleasant to eat. Cut the leeks in half the long way. Place the leeks on their flat sides and slice them into thin half-moons. Add them to a salad spinner (or bowl) with water. Scrunch and agitate them to separate the layers and clean them. Spin or drain and pat dry.
Heat oil and butter. Add onion, leek, and thyme. Season with salt and pepper 8-10 minutes, stirring often, or until the onion and leek are soft and browned.
Roll the pastry sheet to 10" x 15". Dock the center of the pastry with a fork. Brush with egg wash and bake for about 10 minutes or until lightly browned all over.
Spread the onion and leek mixture in an even layer, avoiding the outer half inch. Sprinkle the cheese in an even layer. Return to the oven for 5 minutes or until the cheese has melted. Slice and serve.
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Notes
This tart is best served right when you make it, but I have saved leftovers in the fridge and air-fried them to reheat. The bottom doesn't get crispy again but it's still delicious.