Place the cashews in a bowl and cover with freshly boiled water. Let sit while you prepare the vegetables for the dip.
Melt the vegan butter in a large skillet over medium heat, then add the minced garlic and cook for about 1 minute, stirring frequently, until fragrant. Add the chopped spinach and artichoke hearts and cook for another 3 or 4 minutes, stirring frequently, to infuse the vegetables with garlicky butter. Turn off the heat.
Drain the soaked cashews and place in a blender with 1 ½ cups water, the nutritional yeast, miso, lemon juice, cornstarch, and salt. Blend on high speed until smooth and creamy. Pour the cashew mixture over the garlicky vegetables in the skillet, stir, and cook over medium heat until the sauce thickens and is bubbling. Stir in the shredded vegan cheese until melted. Taste and adjust salt and pepper if needed.
The dip can be served as is or baked for a thicker, cheesier dip. To bake, pour the dip into a lightly oiled 8-inch square casserole dish, top with the ¼ cup shredded vegan cheese, and bake at 375°F for 20 minutes until bubbling. Either way, serve the dip hot, with chips or pita wedges for dipping.