In a large dutch oven, melt butter over low temperature on stove top and then add onion.
Cook over low heat for about 15 minutes until onion is soft but not browned
Add zucchini, cover, and cook for 10 additional minutes, stirring occasionally.
Add salt, pepper and nutmeg and stir to combine.
Sprinkle flour over zucchini mixture and then slowly add milk and stir to combine.
Cook for a few more minutes until thickened and then pour into a buttered 8 x 8 casserole dish.
Combine cheese and breadcrumbs and use to top zucchini.
Bake for about 20 minutes until golden brown.