Place peeled quartered potatoes and peeled cubed zucchini with seeds removed if they are large ones into a large stock pot cover with water and lid then bring to a boil. Let simmer for approximately 20-25 minutes until fork tender then drain.
In second pan melt butter and add garlic, rosemary & milk and heat gently for a few minutes.
Add sour cream & 1/2 of the liquid mixture into the drained zucchini & potatoes mashing (I use an electric beater or kitchen aid mixer) Blend until they become smooth adding more liquid if needed. Be careful not to over do it. When they reach the right consistency you may season with salt & pepper and stir in the parmesan cheese then serve.