Preheat the oven to 425º F.
Add cauliflower and peeled garlic cloves to a baking sheet. Drizzle with 2 tablespoons olive oil, and generously season with salt and pepper.
Roast cauliflower and garlic in the oven for 30 minutes, until the cauliflower has started to brown around the edges.
While the cauliflower is roasting, add the additional 1 tablespoon of olive oil to a large soup pot, and saute the diced onions for 12-15 minutes over medium heat, stirring to prevent over browning. The onions should be a light caramel color, but too deep of a color will make a brown instead of white soup.
Add water and vegetable bouillon to the pot, along with the cashews and roasted cauliflower and garlic.
In batches, process the soup in a blender. Process on high speed until the soup is very creamy. I typically let my high speed blender run for 2-3 minutes per batch for a super smooth soup.
Return soup to the pot, and season with thyme, lemon juice and salt and pepper, if desired.
Refrigerate leftovers for up to 3 days, or freeze in sealed containers for up to 3 months.