Preheat the oven to 200°C, fan 180°C, gas 6. Put the squash, garlic and half the olive oil into a bowl and toss together. Place two large sheets of foil on a work surface and divide the mixture between the two. Season. Fold up the sides of each piece of foil to seal in the squash and seal tightly to make two parcels. Transfer to a baking tray and bake in the oven for 30 minutes.
Meanwhile, put the balsamic vinegar, mint, red onion and remaining olive oil in a non-metallic bowl and season. Add the lamb cutlets and turn to coat, ensuring the cutlets are covered in the marinade. Set aside for 10 minutes to marinate.
Preheat the grill at a high setting. Remove the foil parcels from the oven and carefully open them up. Return to the oven and roast for a further 10 minutes. Meanwhile, grill the lamb cutlets for 12-15 minutes, turning halfway, until starting to char and cooked to your liking. Transfer the squash to a bowl and mash until smooth. Stir in the parsley and season to taste.
Place the spinach in a large pan, add a splash of water and gently heat until wilted. Season. Spoon the mashed squash on to warmed plates, top with some spinach and the lamb chops along with any pan juices.