Preheat oven to 450 degrees. Toss cauliflower and broccoli on baking sheet with the EVOO, salt, pepper and garlic. Roast approx 25 minutes or until spotted with brown.
Melt butter in soup-sized pan over medium-high heat. Saute onions until soft. Add cauliflower, broccoli, water, broth, seasonings and bring to boil. Reduce heat and simmer for 5 minutes.
Puree large pieces of soup and return to pan. Depending on what consistency you want, you may decide to just puree half of the veggies and leave the others. Reheat if necessary. Top soup with shredded cheese and green onions.