Prepare the polenta by bringing vegetable broth to a boil, then whisk in the polenta and stir until thickened.
Cook the spinach and onion mixture by sautéing diced onion in olive oil, adding garlic, and then stirring in chopped spinach and Italian herbs.
Assemble the casserole by layering half of the polenta mixture, the spinach mixture, and the remaining polenta and cheese in a greased baking dish.
Bake in a preheated oven at 375°F (190°C) for 25-30 minutes until golden brown and bubbly.
