Melt the butter in a large pan over medium heat. Add the GF bread crumbs, chopped cashews, and julienned leek green tops. Season generously with salt, pepper, and 1/8 teaspoon of nutmeg. Toast the mixture, stirring often, until golden brown and fragrant. Remove from the pan and set aside.
Brown the 2 Tbs of butter over medium heat. Add the 2 Tbs of olive oil and the sliced leek. Season well with salt and pepper. Sauté until the leeks are softened. Add the sliced garlic and cook for 1 minute until fragrant. Add the 4 cups of cauliflower and 1.5 cups of broth. Cover the pan and cook for 10 minutes.
Pour in 1 cup of milk and lower the heat to a gentle simmer. In a separate small bowl, mix the second 1 cup of milk with the 2 Tbs of GF flour until smooth. Add the flour and milk to the pan.Simmer gently, stirring occasionally, until the cauliflower is tender and the sauce has thickened.
While the sauce is simmering. Bring a large pot of salted water to a boil. Cook the GF spaghetti according to package directions until al dente. Drain and set aside.
Take the sauce off the heat and add the cheddar and mozzarella cheese until melted and smooth. Gently stir the cooked pasta into the sauce until fully coated. Serve immediately, topped generously with the reserved seasoned bread crumb topping.
