In a small bowl, combine lemon juice with sugar and salt. Add onion and let marinate while preparing the recipe.
Wash and cut cauliflower into florets or steak slices.
Preheat oven to 450 °F (230 °C).
In a large pan on medium-high, heat half the oil and melt the butter. When the butter foams, add half the cauliflower and cook for about 5 minutes per side to brown.
Place the cauliflower pieces on a baking sheet lined with parchment paper.
Brown the remaining cauliflower with the remaining oil and butter.
Transfer to baking sheet and sprinkle with BBQ seasoning to coat all sides.
Bake for 10 minutes.
In an ovenproof dish, place pumpkin and/or sunflower seeds and toast for 3-4 minutes.
Meanwhile, mix sauce ingredients.
Serve hot cauliflower with sauce, pickled onions and sprinkle with roasted pumpkin and/or sunflower seeds.