Preheat the oven to 475 degrees.
In a 10 or 12-inch ovenproof skillet, heat the oil over a medium-high heat. Start boiling 1/3 cup water in a pot, kettle, or microwave.
Add the garlic to the skillet, and sauté until it is fragrant, about 1 minute.
Add the spinach (optional) and sauté until the the spinach has wilted.
Add the tomato paste, oregano, and crushed red pepper flakes (optional) and stir until it is well mixed with the garlic, about 30 seconds. Turn down the heat if the garlic is starting to turn brown/burn.
Add the beans, water, salt, and pepper and stir to combine.
Top evenly with the mozzarella and then move the skillet to the oven. Bake the beans for 10 - 15 minutes, or until the cheese is melted and browned in spots. If you want it even toastier, put it under the broiler for a minute. Serve immediately.