Add the onion, garlic, and 5 tablespoons (75 g) water to a small pan over medium-low heat. Bring to a simmer, and cook 5 to 8 minutes or until tender, stirring occasionally.
Add the spinach to the pan in small batches, stirring and tossing the spinach until it is well cooked and wilted before adding the next batch. Repeat until all of the spinach is cooked well. This will take about 10 minutes. Turn the heat off.
Preheat the oven to 375 degrees F (190 degrees C).
Roughly chop the artichokes into quarters. This will release lots of liquid, so drain the excess. Squeeze out the excess liquid into the sink. You don't have to be obsessive about every drop, but give them a good squeeze so the dip doesn't end up watery. Add the artichokes to the cooked spinach mixture, and set aside.
Add the yogurt, vinegar, cashews, salt and pepper, to a high-speed blender or food processor (see Note). Add the potato to the blender; blend on high until completely smooth. Scrape the sides if needed, and blend once more. Pour over the spinach and artichoke mixture, and stir to combine everything well.
Pour the mixture into a 9-inch cast-iron skillet, pie dish, or an 8-inch square baking dish. Bake at 375 degrees for 25 minutes or until the top looks firm and set and is bubbly. Let cool for 5 to 10 minutes. For a kick, add some red pepper flakes on top before serving with pita chips or veggies.