Preheat oven to 400°F. Grease a large baking sheet or line with parchment paper.
Toss cauliflower, cremini mushrooms, and 3 tablespoons of olive oil together. Spread in an even layer on prepared baking sheet. Roast for 30 minutes, stirring half way through.
In a large sauce pan over medium-high heat, add 1 tablespoon olive oil. Add garlic and let cook for 30 seconds. Add leeks, cooking until softened, about 10 minutes.
Add all the shredded cheeses and milk to the sauce pan and stir, until cheese melts. Then add in the sour cream, ground mustard, pepper and salt. Set aside.
Once cauliflower and mushrooms are roasted, remove from the oven and add to a large bowl. Add the cheese sauce mixture and fold to combine. Transfer mixture to a greased casserole dish. (11x7 or 9x13 will both work.)
In a small bowl, combine the melted butter, bread crumbs and parsley. Sprinkle over the top of the mac and cheese.
Bake for 12 to 15 minutes, just until the bread crumbs on top are lightly browned. Serve immediately!
Video
Notes
Storage: Once fully cooled, refrigerate it in an airtight container for up to 5 days.Reheat: Sprinkle it with 1 teaspoon milk and microwave it for up to a minute. Or, pop it into the oven at 325°F for 15 minutes, until heated through.Do it over low heat. Don’t increase the heat as you make the sauce thinking it will melt the cheese faster. Low heat is the only way to ensure a smooth sauce with no lumps.Use a cheese mix. Don’t worry if you don’t have different kinds of cheddar at home. Any melty cheese mix or bits of leftover cheese will do the trick.Add more toppings. Crumbled bacon, chopped chives, and even grilled sausages will add more flavor to each bite.Add more veggies. Roasted bell peppers, grilled corn, peas, or broccoli florets can add more nutrients and flavor to the dish.Cover it. If the top is browning too quickly, cover the baking dish with foil for the remainder of the baking time.