Cook the pasta according to package directions for al dente, in salted water. Drain, reserving 1/2 cup of the pasta cooking water.
Meanwhile, add the butter to a large pan on medium heat. Once melted, add the rosemary and let it cook for 2 to 3 min, then remove the rosemary using a slotted spoon or tongs. Add the red onion, olive oil, salt and pepper. Stir and reduce the heat to medium low. Cook onions for 8 to 10 min, stirring periodically. A bit of browning is fine, but add a tablespoon of water if needed, to prevent burning. Add vegetable broth and cook until broth is halfway evaporated.
Stir in the pumpkin puree and coconut milk, along with the garlic powder. Bring the sauce to a simmer, then very carefully transfer it to a blender. Add half of the sun-dried to the blender. Carefully blend then pour sauce back into the pan, using a spatula to get as much sauce from the blender as possible.
Add the cooked pasta and remaining sun-dried tomatoes, and stir well. If you want a thinner sauce, add some of the pasta cooking water, 1 to 2 tbsp at a time, until the sauce is how you like it.
Serve and top with shredded / grated vegan parmesan. If you're a rosemary lover, you can finely chop 1/2 tsp fresh rosemary to add into the pasta.