Bring a large pot of water to a boil. Add pasta, and cook according to package directions. Five minutes before you expect the pasta to be done, add green beans and carrots to the pot. Return water to a boil, and continue to cook until pasta and veggies are tender. Drain and transfer pasta, green beans, and carrots to a large serving bowl.
To make the dressing, in a blender, combine the cauliflower, almond flour, nutritional yeast, vinegar, garlic, Italian seasoning, mustard, salt, pepper, and 1/2 cup of water. Blend 5 to 7 minutes or until smooth.
Transfer the dressing to a saucepan and cook on medium heat for 4 to 5 minutes to thicken. Remove pan from heat and let dressing cool.
Add the dressing, mixed greens, tomatoes, and onion to the pasta mixture, and toss to coat.
Garnish with parsley and pepperoncini (if using) right before serving.