In a blender (or food processor) add approx. 1 cup of the cauliflower florets and process for approx 15 sec. until the cauliflower is the consistence of rice. Transfer to a large bowl and repeat until the remaining cauliflower is processed.( It's important to work in small batches when making the cauliflower rice to get even "rice" size pieces).
Add beans and broth to blender and puree until smooth ( approx. 30 sec.-1 min.)
Add olive oil to a large sauce pan (with tight fitting lid) and heat on medium-low (or spray pan with cooking spray). Add mushrooms and onions , stirring frequently, cook until onions are soft and mushrooms are lightly browned (about 7-8 min.). Add garlic, salt and pepper and continue to cook for an additional minute. Add lemon juice and increase heat to medium-high. Continue cooking and stirring until almost all the liquid has evaporated.
Add bean puree to the pan with the mushrooms and bring to a simmer.Stir in the cauliflower "rice" . Reduce heat to medium, cover the pan and cook, stirring occasionally, until cauliflower is tender (approx. 8-10 min.).
Uncover the pan and reduce heat to low. Add the cheese and stir until combined. Turn off the heat and stir in parsley. Divide Risotto among serving bowls and drizzle with truffle oil . Top with additional cheese and parsley if desired.