Cauliflower Steaks

1 large head of cauliflower
1/2 cup pitted oil-packed black olives, finely chopped
3 sun-dried tomatoes, thinly sliced
3 1/2 tablespoons olive oil, divided, plus more
2 tablespoons chopped flat-leaf parsley
1 teaspoon fresh lemon juice
Kosher salt and freshly ground black pepper
3 garlic cloves
2 plum tomatoes, cored, quartered

Remove leaves and trim stem end of cauliflower, leaving core intact. Place cauliflower core side down on a work surface. Using a large knife, slice cauliflower into four 1/2" "steaks" from center of cauliflower (some florets will break loose; reserve). Finely chop enough loose florets to measure 1/2 cup. Transfer chopped florets to a small bowl and mix with olives, sun-dried tomatoes, 1 tablespoon oil, parsley, and lemon juice. Season relish with salt and pepper.
Preheat oven to 400°. Heat 1 tablespoon olive oil in a large heavy ovenproof skillet over medium-high heat. Working in 2 batches, cook cauliflower steaks until golden brown, about 2 minutes per side, adding tablespoon oil to pan between batches. Transfer steaks to a large rimmed baking sheet. Reserve skillet. Roast cauliflower until tender, about 15 minutes.
Meanwhile, return skillet to medium-high heat and add garlic cloves and tomatoes, one cut side down. Cook until tomatoes are browned; turn tomatoes over and transfer skillet to oven with cauliflower. Roast garlic and tomatoes until tender, about 12 minutes.
Transfer garlic, tomatoes, and 1/2 tablespoon oil to a blender; purée until smooth. Season with salt and pepper. Divide tomato sauce among plates. Place 1 cauliflower steak on each plate; spoon relish over. Serve warm or at room temperature.