Preheat your oven to 350°F (180°C).
Dice the broccoli stems and separate the florets. Boil the florets in a saucepan for 2–3 minutes, until just tender but still bright green. Drain and set aside.
In a Dutch oven or large pot, melt the vegan butter over medium heat. Add the leek and sauté for 2 minutes until softened. Add the chopped garlic and diced broccoli stems; cook for another 2 minutes, stirring frequently.
Stir in the soaked cashews and spinach. Cook until the spinach has wilted. Sprinkle in the flour, onion powder, paprika, nutritional yeast, salt, and pepper. Stir well to coat.
Gradually pour in the vegetable broth and soy milk, stirring constantly to avoid lumps. Add the miso paste and apple cider vinegar. Simmer for 5–10 minutes, stirring often, until thickened.
Carefully transfer the sauce to a high-speed blender and blend until completely smooth and creamy. (You may need to do this in batches.)
Return the sauce to the pot. Stir in the cooked macaroni and reserved broccoli florets. Mix well, then smooth the mixture into a lightly greased 9x13-inch baking dish.
In a medium bowl, mix together the panko, shredded vegan cheese, hemp seeds, olive oil, nutritional yeast, and salt. Sprinkle evenly over the pasta.
Transfer to the oven and bake for 30 minutes, or until golden and bubbling. Serve hot.