Place a large (10 inch wide) nonstick skillet that is oven safe on the stovetop. Heat over medium heat.
Add the oil to the skillet. When the oil is hot, add the chopped spinach. Cook until the spinach is just tender, approximately 3-5 minutes.
While the spinach cooking, in a small bowl, whisk together the eggs, cheese, salt, and black pepper. Chop the artichoke hearts.
When the spinach is just tender, add the artichoke hearts and stir to combine. Then add egg mixture to the skillet. Stir briefly. Let the egg mixture cook until the edges just begin to set, approximately 3-5 minutes.
Turn off the stove and place the oven safe skillet in the oven. Bake until the eggs are completely cooked through and the frittata is dry to the touch on top, approximately 10-12 minutes.
Remove the frittata from the oven. Run a rubber spatula around the edge of the frittata to loosen it and either slide it out or flip it onto a plate. Serve immediately.
To ensure a truly vegetarian frittata, make sure to use a vegetarian cheese.