Don’t worry about exact amounts for this recipe. If you can only find artichokes or spinach in slightly smaller or larger amounts, that’s fine. In this recipe, frozen is actually better than fresh because the softer texture works better for dip, so take the easy way.
Defrost the vegetables in the refrigerator overnight, then drain them a colander in the sink or over a bowl. You can drain them together in the same colander because they’ll get mixed together anyway, and that makes fewer things to wash. After they are drained, press on them with paper towels to get rid of any extra moisture that would dilute the dip’s flavor.
Preheat the oven to 350. Combine all the ingredients except 2 tablespoons of the Parmesan cheese into a large bowl or into a food processor. Blend or pulse until mostly smooth but leave some artichoke chunks for texture.
Pour the mixture into an oven-proof dish or casserole. Sprinkle with the remaining Parmesan. Bake for 20 minutes, or until heated through and just starting to bubble.
Remove from the oven and let it cool for at least 10 minutes. Serve hot or warm with thin slices of baguette, raw vegetables, baked tortilla chips, or crackers.