For the aioli:
1 clove garlic, thinly sliced
2 tablespoons lemon juice
Pinch of salt
3/4 cup roasted red peppers from a jar, drained and diced
1/2 cup mayonnaise (any kind)
1/4 teaspoon Maras, Aleppo, or other red pepper flakes
For the cauliflower:
2 medium or 1 very large head of cauliflower
3 to 4 tablespoons olive oil
1/2 teaspoon salt
1/8 teaspoon ground black pepper
1 teaspoon smoked paprika
2 large tomatoes, thickly sliced
2 handfuls arugula, or other greens of your choice, such as crisp lettuce
1 Preheat the oven to 450ºF. Spray or brush a rimmed baking sheet with olive oil.
2 Make the aioli: In a food processor, combine the garlic, lemon juice, and a pinch of salt. Let the garlic soak for about 5 minutes to soften the garlic’s raw harshness. Add the roasted peppers and puree until smooth. Add the mayonnaise and red pepper flakes and pulse to mix. The consistency of the aioli should be slightly thinner than mayonnaise. Taste and add more red pepper flakes, if you like.
3 Slice the cauliflower steaks: Leaving the core intact, trim a sliver from the bottom of the stem end of the cauliflower and remove the leaves. Stand the cauliflower upright on its base. With a large knife, cut it into 1/2-inch thick steaks. You should get at least 4 slices that hold together.
You will have some “scraps”—slices of cauliflower that are not attached to the base. If they are flat, you can roast them with the steaks and use them to build your sandwich. Don’t throw other scraps away! Either roast them on a separate baking sheet when you roast the steaks, or save them for cauliflower rice.
4 Roast the cauliflower: Brush both sides of the steaks and the flat scraps with oil. Sprinkle both sides generously with salt and pepper. Set them on the baking sheet, and sprinkle lightly with the paprika.
Cover the cauliflower with foil and roast for 10 minutes. (This allows the cauliflower to steam briefly and soften in the center.) Remove the pan from the oven and remove the foil. Return it to the oven and roast for 10 minutes longer, or until the cauliflower begins to brown.
Carefully turn the steaks over, sprinkle with more paprika, and bake for an additional 8 to 10 minutes, or until golden and tender when you insert the tip of a knife into the steak. Remove from the oven and set aside. Total roasting time is about 30 minutes.
5 Spread the aioli on each plate. Top with arugula, and the cauliflower on top. Top each with tomato slices. Drizzle with more aioli and serve.