If using the oven method, preheat the oven to 425F or 400F convection or you can cook these in a skillet.
In a medium bowl, add the spinach, artichoke, herbs (if using), cheeses, garlic, pepper, and lemon zest (if using). Use your hands to mix well to combine.
Brush the outside of each slice of bread with oil or spread on a thin layer of butter. Flip the bread over, oiled side down.
Scoop the mixture onto the bread (about 1 heaping cup per sandwich). You really want to load it up and pack it down into a compact layer). Top with the other corresponding slice to form your sandwich.
For the crispiest result, you can cook these in a covered skillet over medium heat until crispy and golden on both sides or use the oven method, described below.