Heat a large heavy bottom pot over low heat, add the oil and heat through. Drop one popcorn kernel into the pot and when it pops, add the remaining kernels and cover the pot.
Shake the pan and continue shaking the pan every minute or so that by the end of the cooking process, all the kernels hit the heat evenly and pop, about 5 minutes total.
Remove from the heat and add the popcorn to a large bowl. Evenly pour the melted butter and garlic powder over the top and toss to coat.
Next, add the half of the Parmesan, Pecorino Romano, nutritional yeast and rosemary and toss to coat. Sprinkle the remaining cheese, nutritional yeast and rosemary over the top. Top with the parsley to garnish, if using.