For more vibrant color, pick young dark green zucchini.
Wash and cut zucchini into 1" thick rounds.
Transfer the rounds to a pot and add cold water to cover zucchini to the top. Add salt and toasted garlic and bring to boiling over high heat. Reduce the heat to minimum and simmer until zucchini is soft. It should take about 5 minutes.
Strain and reserve water in case you need to adjust soup thickness. Transfer zucchini into a blender jar, add milk and spices if using, and puree until smooth. Taste and adjust seasoning if needed.
Return the soup to the pot and bring it back to simmer to remove bubbles. Meanwhile, wash and finely chop fresh dill, wash and slice scallions. Warm soup bowls. In each bowl, pour a portion of hot soup, drop a dollop of cold sour cream in the center and top it with salmon roe.