Step 1 Put chopped potato in a large microwave-proof bowl and top with a plate. Microwave on high for 7–8 minutes, or until tender, then roughly mash.
Step 2 Put broccoli and frozen peas in a microwave-proof bowl and microwave on high for 2 minutes. Add mash and pesto to bowl; mix to combine. Form into 8 patties.
Step 3 Spray a medium non-stick frying pan with cooking oil; set over medium heat. Cook patties, turning once, for 10–15 minutes, or until browned on both sides. Transfer patties to a plate and cover to keep warm.
Step 4 Spray pan with oil again. Add lamb steaks and cook for about 2 minutes per side (for medium rare), or until done to your liking. Transfer steaks to serving plates.
Step 5 Meanwhile, place sprouts in a microwave-proof bowl; steam for 2–3 minutes. Add sprouts to pan; stir-fry for 2–3 minutes, or until slightly browned. Divide sprouts among serving plates and drizzle with balsamic vinegar.
Step 6 Serve each steak with 2 potato cakes and Brussels sprouts.