Step 1Heat oven to 350 degrees. Line a baking sheet with parchment paper. Tear up the bread slices and put in the bowl of a food processor. Pulse the bread until it is reduced to coarse crumbs.
Step 2In a bowl, combine the bread crumbs with the garlic, olive oil and salt and pepper to taste. Mix well to get the olive oil over as much of the crumbs as you can.
Step 3Spread the seasoned bread crumbs on the prepared sheet and bake until they are golden brown, 10 to 20 minutes. Halfway through baking, stir the crumbs around so they bake more evenly. Be careful not to let them burn. Set aside.
Step 1Heat the oven to 450 degrees. Line a rimmed baking sheet large enough to hold the cauliflower slices with parchment, and drizzle the parchment with olive oil (this will help to color the bottom of the slices).
Step 2Trim the stem end of the cauliflower without cutting out all the core. You need the core to keep the florets together. Cut the head into steaks as thick or thin as you choose. Don’t worry if some florets fall apart; you’ll just add them to the baking sheet.
Step 3Lay the steaks out onto the prepared sheet pan and add the broken florets to the empty spaces of the pan. Drizzle more olive oil over the cauliflower. Sprinkle over salt to taste, along with a bit of chile flakes.
Step 4Bake the cauliflower steaks on the lowest rack of your oven until they just begin to become tender, 10 to 15 minutes, depending on thickness.
Step 5Carefully flip the slices over and bake until tender and golden brown (a knife should pierce easily), 10 to 15 minutes more.
Step 6Top the slices with the tomato sauce and about half of the Parmigiano. Return to the oven and continue to bake until the sauce is warmed and the cheese is melted, about 5 minutes.
Step 7While the cauliflower is baking, toss the bread crumbs with the olives, capers and anchovy, if using, along with the remaining cheese and parsley. Top the cauliflower steaks generously with the bread crumb mixture, then push the larger individual florets together, and top them with bread crumbs too. Return to the oven and bake until the bread crumbs are warmed, 2 to 3 minutes.
Step 8Remove from the oven and serve immediately.