Step 1 Spray a large saucepan with oil and place over medium heat. Cook onion, carrot and celery, stirring occasionally, for 6–8 minutes or until vegies are softened. Add garlic and cook, stirring, for 1 minute more. Add mince and cook, stirring, for 5 minutes, or until browned.
Step 2 Add tomato paste and cook, stirring, for 2 minutes. Add tomatoes and ¼ cup water. Cook, stirring occasionally, for 10 minutes, or until sauce thickens. Remove and set aside.
Step 3 Preheat oven to 180°C. Lightly spray an 8 cup-capacity (25 x 18 x 6cm) ovenproof dish with oil. Place sweet potato into a large microwave proof dish with 2 tablespoons water. Cover and microwave for 4–5 minutes on high, or until just tender. Drain.
Step 4 Whisk ricotta, egg and milk in a large bowl until smooth.
Step 5 To assemble lasagne, spread 1/3 of sweet potato over base of prepared dish to cover. Top with 1/2 the mince mixture. Repeat with another layer of sweet potato and remaining mince. Finish with a layer of sweet potato, then spread with ricotta mixture. Bake for 40–45 minutes, or until top is golden and bubbling. Sit lasagne for 5 minutes before serving slices with baby spinach.