Preheat oven to 375°F. Grease a 9"x13" baking pan or casserole.
In a large skillet, melt 2tbsp butter and 2tbsp oil on medium heat. Add mushrooms and onions and cook, stirring occasionally, until the veggies are tender and starting to lightly brown.
Stir in flour, half-n-half, thyme and 2tbsp chives. Cook for 3~4 minutes.
Next add the grated cheese and cook for 2 more minutes.
Add the green beans, corn, salt and pepper. Mix well and turn off the heat.
Transfer the mixture to the prepared baking dish.
Melt 2tbsp butter and combine with the bread crumbs and parmesan cheese.
Sprinkle the breadcrumb mixture evenly over the green beans. Bake for 30 minutes or until the casserole is hot and bubbling. Let cool for 10 minutes before digging in.