Preheat oven to 190C/375F. Break cauliflower into florets and toss in a large bowl with olive oil. Season with salt and pepper and spread out on a large baking sheet lined with parchment paper. Place in oven and cook until tender and golden, approximately 35minutes.
Meanwhile, dice the bread. Place in a shallow pan and drizzle with olive oil and sea salt. Put in oven next to cauliflower. Bake until toasted and crisp, about 15 minutes.
Mash the garlic in a mortar with a pestle. Pour in lemon juice and let sit for 10 minutes. Coarsely chop the achovy fillets and then add them to the mortar and pound to a paste. Drizzle in the olive oil while stirring with the pestle until the sauce has become thick yet pourable.
Place the roasted cauliflower on plates, drizzle with anchovy sauce, sprinkle with chopped parsley and croutons, and serve, either warm or at room temperature.