Place a large saucepan over medium heat, and add the sausage, diced yellow onion and olive oil. Cook, stirring often, until the sausage browns and the onion is soft, about 5-8 minutes.
2 Tablespoons Olive oil, ½ lb. Italian sausage, ½ cup Yellow onion
Add the minced garlic, crushed tomatoes, water, red pepper flakes, and Italian seasoning, and stir well. Add in salt and pepper to taste, then bring to a boil and simmer, covered, for 15 minutes.
½ teaspoon Garlic, 14.5 oz. Crushed tomatoes, ⅓ cup Water, 1 teaspoon Italian seasoning, ¼ teaspoon Red pepper flakes, pinch Salt and pepper
Preheat the oven to 350°F. While it's heating, combine the egg and ricotta in a mixing bowl.
1 Egg, 15 oz. Ricotta cheese
Remove the meat sauce from the heat, and use it to cover the bottom of a deep baking dish with a thin layer.
Top the layer of sauce with a 1-sheet-thick layer of lasagna, then ⅓ of the ricotta and egg mixture, then a layer of Mozzarella. Repeat three more times.
1¾ cups Mozzarella, 3 sheets Lasagna
Cover the lasagna pan with foil, then bake for 30 minutes. Remove the foil, bake another 10 minutes, then serve and enjoy!