Combine the spices in a small bowl and rub spice mixture into both sides of the steaks. Set aside for 30 mins to come to room temperature.
Preheat a heavy-based pan over medium-high heat. Brush steaks with oil and season with a good pinch of salt. Cook on preheated pan for 3 mins, flip and cook for a further 2-3 mins. Transfer steaks to a plate and cover with foil. While steaks rest, add onions to the same pan with a splash of oil, cook 4-5 mins, until starting to soften and caramelise.
Combine greens, tomato, cucumber and avocado, drizzle with balsamic vinegar and olive oil, season with salt and pepper. Serve steak with Dijon mustard and salad.