Heat the wok until very hot, add the oil and the non-dairy butter and swirl it around to coat the sides. Stir-fry the fennel slices and garlic for 2 minutes, or until they begin to soften. Add the lemon juice and tomatoes, and season with salt. Stir-fry for 2 minutes, then add the oregano and olives, and toss gently to combine. Place in a serving dish and crumble feta cheese over the top. Sprinkle fresh black pepper.