Preheat your oven to 425°F (215°C). Lightly coat the flesh of the butternut squash with cooking spray or a little olive oil and place it flesh-side down on a baking sheet.
Roast until tender when pierced with a fork, about 40 minutes.
Let it cool until it's comfortable to handle.
Scoop the roasted squash flesh from the skin and transfer it to a food processor.
Add 1 tsp (5 ml) of olive oil, the chopped garlic, fresh thyme, nutmeg, and a pinch of salt and pepper.
Blend until the mixture is completely smooth.
Set your butternut squash sauce aside.
While the squash roasts, heat the remaining 1 tsp (5 ml) of olive oil in a large skillet over medium-high heat.
Add the sliced onion and herbes de Provence, cooking until the onions begin to brown, about 3 minutes.
Reduce the heat to medium-low and continue to cook, stirring occasionally, until they are deeply caramelized and sweet, about 20 to 25 minutes.
In a separate large, dry skillet over medium heat, add the sliced Portobello mushrooms in a single layer.
Cook, stirring only occasionally, until they are soft and golden brown, about 3 to 5 minutes.
Season with a pinch of salt and pepper.
Prepare your pizza base by stretching or rolling the dough to fit a 15 or 16-inch pizza stone or baking sheet. Lightly dust the stone or sheet with flour or cornmeal to prevent sticking. Lay the dough on top.
Spread the butternut squash purée evenly over the dough, leaving a ½-inch border for the crust. Scatter the caramelized onions and cooked mushrooms over the sauce.
Add dollops of the Yogurt Cheese and finish with a sprinkle of chopped hazelnuts.
Bake for 15 to 20 minutes, or until the crust is golden brown and the toppings are heated through.
Remove the pizza from the oven, transfer to a cutting board, slice, and serve immediately.
Don't overcrowd the pan when cooking the mushrooms.
Cook them in batches if necessary to ensure they brown properly instead of steaming.
