Preheat oven to 400 degrees and grease a 9x13 baking dish. Melt butter in a skillet over medium heat, and cook onion until tender. Spread cheddar cheese in bottom of prepared baking dish. Layer with mushrooms, olives, sauteed onion, ham, and jalapeno peppers. In a bowl, scramble eggs together with milk, and season with salt and pepper. Pour egg mix over ingredients, but do not stir. Bake uncovered in the oven for 30 min or until no longer runny in the center and slightly brown on top. Allow to cool, then cut into squares and serve.