Easy Rice Bake Casserole

2 tablespoons butter, plus more for dish

1 small onion, minced

Pinch cayenne pepper

1 package frozen spinach, thawed, drained and chopped

1 cup milk

2 eggs

2 cups shredded sharp Cheddar, plus extra for top

4 cups cooked rice

1 teaspoon freshly chopped parsley leaves

1 teaspoon freshly chopped thyme leaves

1 teaspoon chopped basil leaves

Salt and freshly ground black pepper