To make the relish:

Preheat the oven to 425 degrees and prepare a sheet tray with a roasting rack on top.

Slice the plum tomatoes in half and lightly coat with olive oil, season with salt and pepper and place cut side up on the roasting rack. Lightly coat the red bell peppers with olive oil and place on the same roasting rack. Cook in the oven for 35 minutes.

After roasting, place the peppers in a bowl tightly covered with plastic wrap or in an airtight container to allow the steam to loosen the skin. Let the tomatoes cool on the rack.

When the tomatoes are cool enough to handle, peel the skin off and discard. Slice the tomatoes in thin strips and set aside.

When the peppers are cool enough to handle, peel the skin and discard it along with the seeds and stem. Slice the peppers in thin strips and set aside.

Finely chop the canned tomatoes and set aside.

Slice the olives and set aside.

Finally, place all the ingredients in a medium mixing bowl and mix until fully incorporated. Check for seasoning and add more salt if desired.

To make the cauliflower steaks:

Preheat the grill to high heat (450°F-500°F) making sure the grates are clean and lightly oiled.

Prepare the cauliflower by removing any outer leaves and trimming the stem end to sit flat. Slice the head of cauliflower down the middle of the stalk to create two halves. Trim each half into 1.5 to 2-inch thick steaks. Reserve any florets that break off. Drizzle both sides of each cauliflower steak with olive oil and generously season with salt and pepper, ensuring even coverage. Place the cauliflower steaks directly on the grill grates and cook for about 5-7 minutes on each side until tender and charred. Test for tenderness with a fork or knife. 

To serve:

Serve the grilled cauliflower steaks immediately after cooking. Top with a couple heaping spoonfuls of the olive-red pepper relish, and drizzle some of the oil from the relish around the plate. Optionally, garnish with chopped herbs.