Preheat the oven to 425°F. Slice the cauliflower top to bottom into 3/4 inch thick slices and drizzle olive oil on both sides. Season with salt. In a small bowl, mix 2 tablespoons of harissa paste and 2 tablespoons of olive oil. Coat all sides of the cauliflower liberally with the sauce.
Bake for 35 minutes, carefully flipping after the first 20 minutes.The cauliflower should be nicely golden with a slight char on both sides.
Combine creamy avocado sauce ingredients in a blender, blend until smooth and creamy, and set aside. If the sauce is too thick you can add a pinch of cream, water or extra lemon juice.
Season salmon filets with salt on both sides. Heat 2 tbsp of butter or oil over medium-high heat in a stainless pan or skillet. Once hot, add salmon filets and sear for 3-4 minutes per side for medium rare and 4-5 minutes per side for medium, depending on thickness.
Transfer salmon filets to a plate over the cauliflower and top with creamy avocado sauce. Sprinkle with fresh basil and serve immediately with a lemon or lime wedge.