Slice the cauliflower lengthwise into 1 cm thick steaks. Set aside.
Roast the Peppers. Char the green pepper and jalapeños directly over the burner until the skin blackens. Let cool, then peel and remove seeds.
Toast pumpkin and sesame seeds on a hot pan for a few seconds. Set aside.
Make the Pipian Sauce. Remove stems and chop the peppers. In a blender, combine roasted peppers, jalapeños, spinach, coriander, pumpkin seeds, sesame seeds, onion, garlic, salt, and 1 cup of water. Blend until smooth.
Heat 2 tablespoon vegetable oil in a saucepan over low heat. Add the blended pipian sauce and cook gently for 15 minutes.
Cook the Cauliflower. Season cauliflower steaks with salt and a little oil. Fry on a hot griddle pan for 3 minutes per side until golden and tender.
To serve, place a cauliflower steak on a plate, top with pipian sauce, and sprinkle with pumpkin seeds. Serve with Mexican rice and a refreshing agua fresca.
