Preheat the oven to 350 degrees F (175 degrees C).

Cook bacon in a skillet over medium heat until crispy, 5 to 10 minutes. Transfer to a plate lined with a paper towel until cool; crumble bacon into a bowl.

Melt butter in another skillet over medium heat. Add celery and bell pepper; cook until beginning to get soft, about 3 minutes. Add onion and cover the skillet. Cook over low heat until very soft, 5 to 10 minutes more.

Combine mayonnaise and cream in a large bowl. Add salt, black pepper, and garlic powder and stir to mix. Add crumbled bacon, green beans, and Cheddar cheese and toss to distribute sauce. Pour mixture into a 2-quart casserole dish.

Bake in the preheated oven until bubbly around the edges, about 30 minutes. Cover with a lid or aluminum foil if the top begins to get too brown.