Prepare homemade rough puff pastry dough through 2nd refrigeration. If using store-bought frozen puff pastry, make sure it’s thawed. Keep either dough in the refrigerator until step 4 below.

Over low-medium heat in a large skillet, cook oil and butternut squash together for 5 minutes stirring occasionally. Add mushrooms, onion, garlic, salt, pepper, thyme, rosemary, and nutmeg. Cook until vegetables are soft, about 5-6 minutes. Remove from heat.

Preheat oven to 400°F (204°C). Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.

On a lightly floured work surface using a lightly floured rolling pin, roll pastry dough into a 10×16-inch rectangle. (Tip if using store-bought—place the edge of one sheet over the other and use a rolling pin to adhere them together. Roll the whole thing out into a 10×16-inch rectangle.) Carefully transfer dough to lined baking sheet. If dough has lost shape, use your hands to reshape into a rectangle. Fold over a 1/2 inch edge and crimp the edges down with a fork. Crimping is much easier the colder the pastry is, so place dough/baking sheet into the refrigerator to chill for 10 minutes if needed.

Brush egg wash all over puff pastry including the edges. Use a fork to poke holes all over the dough (not the crimped edge). Sprinkle with 3/4 cup (about 95g) cheese. Spoon vegetable topping over cheese and arrange in a single layer as best you can.

Bake tart for 30 minutes. Remove from the oven and sprinkle remaining cheese all over the top. Return to the oven and bake for 5-8 more minutes or until cheese is melted and pastry is golden brown.

Remove from the oven and, if desired, garnish with flaky sea salt, a sprinkle of pepper, and/or extra cheese and herbs. Slice and serve warm or at room temperature.

Cover and store leftovers in the refrigerator for up to 5 days. Reheat in the microwave or cover and bake in a 300°F (149°C) oven for 10-15 minutes.