Preheat oven to 450 degrees. Gather garlic, anchovies (if using), and red-pepper flakes into a pile. Using a chefs knife, coarsely chop; season generously with salt. Using flat side of knife blade, mash mixture into a paste.
Place paste into a large bowl; add oils, and mix to combine. Add cauliflower, and toss to coat. Spread mixture in a single layer on a large rimmed baking sheet. Roast until starting to soften, about 15 minutes.
Season salmon with salt and pepper. Remove baking sheet from oven; push cauliflower to sides, and place fillets in the center. Arrange two half-slices of lemon on each fillet. Return to oven; bake until fish is opaque throughout, 10 to 15 minutes. Serve with lemon wedges.