In a large pan over medium-high heat add the olive oil and sauté the onions, peppers, and garlic. Cook until vegetables are tender and slightly browned.
In a large mixing bowl stir together chicken, sautéed vegetables, corn, spinach, mayonnaise, and salt and pepper until well combined.
Spread chicken mixture into a large greased casserole dish. Top with shredded cheese and crumbled cooked bacon and bake for 30 minutes, or until cheese becomes bubbly. Garnish with sliced green onions.
Make Ahead: This casserole can be prepared and stored in the refrigerator up to two days in advance, but wait to bake the bacon and to bake the casserole until the day of serving.