Scrub the sweet potatoes well, slice them into rounds of equal thickness, then steam for 12-15 minutes until tender.
Remove from the heat and let cool slightly, then transfer to a blender with the drained and rinsed cashews, steaming water, miso, nutritional yeast, lemon juice, mustard, olive oil and salt. Blend on high until completely smooth, then taste and adjust seasoning as desired. Remember that the sauce will be poured over pasta and will taste less intense in the final dish, so amp up the flavour if need be!
While the sweet potatoes are steaming, cook the penne in salted water until al dente. Drain well and pour the sweet potato sauce over top and mix very well to combine. Fold in the spinach and 1/4 cup / 20 grams of the grated cheese.
Transfer the pasta to an ovenproof baking dish, scatter the top with the remaining cheese, and sprinkle with the sunflower seeds. Place in a 400°F / 200°C oven for 10-15 minutes until the cheese is melted and bubbly. Garnish with parsley if desired. Serve immediately, say thank you and enjoy.