Since the dish takes time to bake, you can prep the veggies the day before and keep them in the fridge to speed things up the day you plan to eat the pasta. For the spinach, you can chop in advance but wash only when you're making the pasta.
To begin, cut the mushrooms into pieces (I cut larger ones into 4 to 6 pieces and smaller ones into 2 pieces). Add the olive oil and half the garlic to a pan on medium heat and fry till you can smell the garlic. Add the chopped mushrooms, season with a pinch of salt and some freshly ground pepper. Stir and cook till the liquid evaporates and the mushrooms are lightly browned around the edges. Set aside to cool.
Now chop bell pepper into pieces, about 1/2 inch wide. Then slice the pumpkin, keeping the pieces about 1/8th of an inch thick. They might have varying sizes if you used pre-cut pumpkin, but as long as the thickness is similar, it's ok. Chop the spinach (doesn't have to be too fine, just small enough that it can be stirred into the pasta easily).
Preheat the oven to 175 C. Line a 9 inch springform tin with two large sheets of non-stick baking paper, placing them in opposite directions so the sides are well covered. Leave an overhang so you can easily peel off the paper later. You can use a large baking dish too if you prefer to scoop the pasta out with a spoon and not slice it. Baking time will vary according to the dish.
Now it's time to make the 'sauce'. In a large mixing bowl, add the cream cheese, mozzarella, cheddar, chopped basil and salt and pepper. Stir well till soft and almost combined, then pour in the water. Use a whisk to stir it in to make a creamy, slightly runny, sauce-like consistency (see photos below). There might be some lumps of cream cheese, that's ok. Remove 1/3 cup of the sauce and set it aside.
Now stir in the pasta, followed by all the raw veggies (keep a few pieces of the pumpkin and bell pepper to scatter on top of the dish to get them a little caramelised). Stir very well, making sure everything is coated in the sauce. Add the mushrooms and stir again.
Tip this entire mixture into the prepared dish. Smoothen the top. Pour over the reserved sauce (it keeps the pasta right on top from drying out), then scatter the remaining pieces of pumpkin and bell pepper on top as well.
Fold the baking paper overhang inwards, onto the surface of the pasta and veggies, then cover the pan tightly with foil.
Bake for 1 hour, then check if the pumpkin and peppers have softened enough, then cover and bake for another 20 to 30 minutes as needed till the veggies and pasta are done. Uncover the dish and turn the oven to the broil setting at 150 C for another 15 minutes or so, until the top is golden-brown.
Let it cool in the tin for 30 minutes, then remove the sides of the pan, peel off the paper from the sides, and slice into wedges. Dig in while it's warm! Store leftovers in the fridge for 2 to 3 days, reheating in the oven as needed. Happy baking!