~2 TB dried herbs (use Italian blend, or a mixture of dried oregano, basil, thyme, and rosemary).
Preheat campfire coals until very hot. Once coals are hot, set aside around 15 coals aside for easy access (you'll be setting them on top of your dutch oven).
Mix ricotta, 1 1/2 cups mozzarella, spinach, salt, and herb mixture together. Taste and add more seasoning if necessary.
Spread 1/3 mixture in the bottom of cast iron dutch oven. Top with 1/3 pasta sauce.
Add one layer of lasagna noodles-- break them as necessary to fit into the round pot. Repeat twice with remaining spinach cheese mixture, pasta sauce, and noodles (you may have some noodles leftover).
Pour reserved liquid from spinach can into a measuring cup, and add water until you have 3/4 cup of liquid total. Pour liquid into lasagna along the edge. Top mixture with remaining mozzarella cheese.
Place dutch oven lid on upside down. Set oven directly on the main batch of coals. Using tongs, set reserved coals on top of lid, and let it bake for about 25 minutes.
Next, remove lid, and leave the dutch oven on the fire uncovered for about 5 more minutes. The excess liquid should be boiled off. If needed, let it cook a few more minutes until any extra liquid has thickened.
Remove dutch oven from heat (if your campfire ring has a grate, you can pull the grate down and set the oven on that). Allow lasagna to cool slightly before serving.