Preheat your oven to 400F and move the rack to the top third of the oven.
Melt the butter in an oven-safe skillet over medium heat, then add the onion and sauté for 5 minutes.
Turn off the heat, then stir in the bruschetta.
Place the feta block on top, and then transfer the skillet to the oven. Bake for 25-30 minutes. The feta should be soft and may be starting to lightly brown on top.
Meanwhile, boil a large, salted pot of water for the pasta. Cook the bucatini al dente. Important: reserve about 1/2 cup of the hot pasta water prior to draining the pasta.
Take the skillet out of the oven and give it a good stir, then stir in the parmesan cheese and fresh basil. Add in the spinach and then pour in a couple of tablespoons of the pasta water and toss until the spinach has wilted, gradually adding more pasta water as needed to get the consistency you like.
Add the drained pasta to the skillet and toss. Season with salt & pepper as needed, and serve immediately.