Preheat the oven to 375°F (190°C) and butter a 13-inch, 3-inch deep round or a 13 x 8 x 3-inch long pan.
Whisk the vegan egg, water, plant milk, and nutmeg in a medium bowl until fully combined and set aside.
Heat the olive oil in a large skillet on medium-high heat. When the oil is hot, add the sausage and cook for approximately 8 minutes, stir frequently, or until they’re browned on both sides. Remove from the pan and set aside.
Add the onions to the same pan you cooked the sausage in and cook for three minutes. Stir frequently. Add the bell pepper and cook for five minutes, stir frequently. Add the garlic and salt and pepper and cook for two minutes. Remove from the pan and set aside.
Add the spinach to the same pan and cook until it’s wilted. Approximately 3 minutes. Make sure to toss.
Remove the spinach from the pan and chop on a clean cutting board. This will ensure that the excess water in the spinach is removed and it won’t make the casserole soggy. To remove the remaining water, place the chopped spinach on a paper towel.
In the buttered pan, spread half of the cubed bread evenly on the bottom of the pan. Sprinkle half of the spinach evenly over the bread. Sprinkle half of the sausage evenly over the spinach. Sprinkle half of the onion and pepper mixture evenly over the spinach. Sprinkle half of the herbs evenly over the onion and pepper mixture. Sprinkle half of the parmesan evenly over the onion and pepper mixture. Repeat with the remaining ingredients.
Give the egg mixture one more whisk and pour evenly in a spiral motion over the casserole to ensure total coverage.
Bake uncovered for approximately 30 minutes, or until golden brown on top.
You can prepare this the night before. Store covered in the refrigerator until ready to bake.