In a medium skillet set to medium-high heat, add olive oil. Once hot, add sausage, crumbling with a wooden spoon as it cooks. Cook until golden brown, about 5 minutes.
Place English muffin cubes in the bottom of a greased 8 x 8-inch baking dish. Drizzle with melted butter. Top with sausage, onion and bell pepper.
In a large bowl, whisk shredded cheddar, eggs, milk, salt and pepper. Pour over top of casserole. Cover baking dish and refrigerate overnight.
When ready to bake, remove from refrigerator 30 minutes before baking. Preheat oven to 350 degrees F. Bake uncovered for 45-50 minutes, or until a knife inserted in the center comes out clean. Sprinkle with green onions. Let stand for 10 minutes before serving.