In bowl or jug, soak cashews in 1 1/2 cups water and set aside while you prepare rest of ingredients.

Preheat oven to 425 F.

In medium-sized bowl, combine cauliflower with 2 Tbsp olive oil, 1/4 tsp salt, and pepper, tossing well to coat. Pour cauliflower onto baking sheet and roast in preheated oven for 25 to 30 minutes, or until fork tender.

While cauliflower is roasting, to medium saucepan, add potatoes, garlic, thyme sprig, and 2 cups cold water. Set on medium heat and bring to gentle simmer. Once contents begin to simmer, cook, uncovered, for approximately 30 minutes, or until potatoes are fork tender, reducing heat if necessary to keep contents at a low simmer.

Remove cauliflower from oven, place on top of soup without incorporating; cover with lid and cook for approximately 10 minutes. Meanwhile, to blender, add cashews and their soaking liquid and liquefy until smooth. Add contents of soup pot and continue to blend to a smooth texture. Taste and add remaining 1/4 tsp salt, if required.

In small bowl, combine remaining 2 Tbsp olive oil, 1 tsp thyme, lemon zest, and Parmigiano Reggiano (if using).

Ladle soup into bowls and drizzle with lemon thyme oil just before serving.