First, make the béchamel sauce.
In a large nonstick pan, melt the vegan butter and olive oil together over medium-high heat.
Add in the flour, all at once, and whisk to incorporate. Allow the flour to cook for 3-4 minutes, continuing to whisk and being careful that it does not burn.
Pour in the cashew milk, a bit at a time, being careful to whisk out any lumps. Remove the béchamel from heat, and add in the salt, garlic powder, pepper, nutmeg, and nutritional yeast. Whisk until smooth and creamy.
Add in the fresh and frozen greens, and return pan to medium heat.
Cook the mixture, stirring occasionally, until greens are wilted and cooked through. Add more salt and pepper to taste. Serve immediately!