Sauté spinach leaves in a little olive oil until they wilt. Season with salt and pepper and set aside. Reserve the pan.
Heat olive oil in the same pan and sauté onions and garlic until translucent. Add tomatoes, salt, and pepper and cook on a high heat until tomatoes soften, about 2-3 minutes. Transfer to a blender jar and set aside to cool slightly.
Place the pepper directly on the open flame of a gas stove. Roast until it blisters and chars on all sides, rotating occasionally with tongs. Place pepper in a zip-lock bag or glass bowl covered with cling film and let it sit for 5 minutes.
Take the pepper out and peel away the charred skin; slit lengthwise and remove seeds. Transfer the fleshy bits to the blender jar along with the onions, tomatoes, garlic and extra virgin olive oil and blend to a coarse purée. Season with pepper and salt and set aside. [This sauce can be made earlier and refrigerated until ready to use.]
Remove the outer leaves of the cauliflower keeping the core intact. Place it core side down on your work surface and using a large knife, slice the cauliflower into 4 thick ‘steaks’ of about 1” each starting from the center.
[You will have small bits breaking off in the process. Save these and any remaining cauliflower for another recipe, like a stir fry.]
Heat a large griddle pan (or regular pan) until very hot. Brush the cauliflower steaks with olive oil and place on the pan, grilling for 2-3 minutes per side. Transfer to a flat baking tray and lay them out in an even layer without overlapping them. Ladle a little of the stock over each steak and season with salt and pepper. Roast until tender but still holding its shape, about 10-15 minutes.
[A good test is to stick a knife in the center of the cauliflower. You want it to go through without too much resistance.]
Divide red pepper sauce among plates. Place 1 or 2 cauliflower steaks on each plate with wilted spinach on the side. Serve warm or at room temperature.