Preheat the oven to 350°F. Heat the olive oil in a skillet over MED-HIGH heat. Add the onions, garlic, mushrooms, and sausage meat. Break up the meat with a spoon and cook it until it has turned brown.
Add the Italian herbs and the red pepper flakes. Stir.
Whisk the ricotta, egg, half the Parmesan, and a pinch of salt until smooth.
In a 9" x 9" casserole dish layer in half the marinara sauce, then a single layer of lasagna sheets, then half the ricotta mix, pepperoni slices, and olives. Repeat the layers, from the sauce to the olives.
Top with the mozzarella balls and the rest of the Parmesan. Bake for 45 minutes, until the cheese is bubbling and golden brown.