Sautee the onions in just a bit of oil, long and slow, occasionally stirring, for about 20 min. Add in zucchini and cook over low fire for about one hour, stirring occasionally. Cover for the first half hour, then uncover.
Put the milk, potato starch and garlic into a small pot and bring to just below boiling, stirring to dissolve any lumps.
When sauce has thickened, add the cheeses and stir until dissolved. Add about four to five turns of black pepper.
Add the zucchini into the cheese sauce and stir until combined.