Cut out the heart of cauliflower and divide the cauliflower into florets. Toss it in a large bowl with some olive oil, salt and fresh cracked black pepper. Spread in one layer on a baking sheet. (You can use the same baking sheet for leeks as well. Mushrooms take a little bit less time, so use another baking sheet.)
Cut off the dark green leaves of the leeks and the very bottom by the roots, discard what you cut off or save it for making homemade vegetable stock. Wash the leeks and cut them length-wise in quarters. Rub leeks with some olive oil and season with salt and pepper. Lay out the leek quarters on the baking sheet next to the cauliflower.
Wash and cut cremini mushrooms in half. Toss them in a bowl with some olive oil, salt and pepper. Spread on a separate baking sheet.
Roast cauliflower and leeks for 25-30 minutes. Mushrooms will need about 15-20 minutes. (Place the pan with mushrooms in the oven after cauliflower and leeks have been roasting for 10 minutes.)
While veggies are roasting, cook Fettuccine in a large pot, drain and set aside.
Use the same pot to make sauce. Preheat the pot over medium heat, add olive oil.
Thinly slice shallots and add them to the pot. Let shallots get to light golden color.
While shallots are cooking, thinly slice garlic cloves. Add garlic to the pot once shallots are lightly golden. Cook both until golden brown.
Whisk in vegetable broth and then slowly whisk in heavy cream. Let the mixture heat through.
Add cheese, salt and pepper and keep stirring until all melted.
Add fettuccine to the pot and mix. Add roasted veggies to the pot and mix well.